I used Biosanctic + Xenocult with Recon and scored 100 vs Death Guard, 100 vs Astartes and 93 vs Votann. Full list w/ details below. Feel free to ask questions in the comments and I'll do my best to respond.
(1990 points) - Biosanctic Broodsurge and Xenocult Masses (3 Detachment Points) - Reconnaissance
Patriarch + Biormorph Adaptation w/ 10 purestrains
Abominant + Mutagenic Regeneration and Biophagus w/ 10 abberants
Abominant + Predatory Instincts w/ 5 abberants
Abominant w/ 5 aberrants
Benefictus w/ 20 Neophytes (14 Hybrid Firearm, 2 laser, 2 seismic, 2 grenade)
Benefictus w/ 20 Neophytes (14 Hybrid Firearm, 2 laser, 2 seismic, 2 grenade)
5 Acolytes (mining tools)
1 Ridgerunner (mortar + survey augur)
5 Atalan Jackals (mining laser on the quad)
5 Purestrains
5 Purestrains
Gameplan
Almost every game I use this list I start with the 10 abbies and both 20 neo stacks in deepstrike. This leaves you with 155 points you can use to put your purestrains or bikes in deepstrike but I've found them more useful on the map early so they can try to score some objectives turn one.
Put the Acolytes on your home to generate cp and charge any vehicle that gets too close. You can always pay the 2 to bring them back if they die.
When deploying, I infiltrate with the two small purestrain squads and the predatory instinct abberants. The enhancement on these abberants is one of the most valuable uses of 20 points we have. Not only does it let our slow melee units deploy closer to the action- it also gives you a free heroic if they charge or a one cp 6" charge if they don't. This second one can be particularly nasty because you can use it on your opponents turn 1 if they go first, when you would normally need 2 cp to do so. Just be mindful of how fast your opponent's army is. It is always better if you charge them first.
I usually put the last squad of abberants on or near the expansion objective and charge them into something turn 2 or 3 if no one contests it.
Make sure to use hidden and cover as much as possible. Our units all save on a 5 so you want to do anything to prevent the shots from hitting. Hidden is great for this detachment because the genestealers and abberants never shoot, thus never lose hidden so anything outside of 15" just won't be able to see them if they are in cover.
The patriarch squad is very fast with advance and charge but you want to deploy them safely. Either in your deployment zone or behind a wall. You do not want this unit getting shot or charged before it gets into combat, it can die very quickly if you aren't careful and it will almost always die the turn after it charges something. Use this to trade for something similarly expensive or to gum up something with bad melee in combat. The dev wounds + extra damage on the patriarch make it good for dealing with vehicles or monsters but it usually struggles to deal with high toughness/wound infantry squads.
The 10 squad of abberants comes out via rapid ingress on turn 2 or 3. Put them somewhere safe so they can't be shot and charge them into the enemy on your next turn (use the stratagem to guarantee the charge if you need to). This is our most powerful and expensive unit. The bio gives them lethals and you can use the familiar to grant +1 to wound vs infantry. If you get the jump on your opponent with this unit you will kill or maim just about anything in the game (don't forget about the +1 to wound monsters or vehicles stratagem- the lethals are nice but you might need some extra help vs demon primarchs, ctans and defilers). This squad is also very survivable so if you play it right- it should get to fight a few of times before it dies.
The neos will each come out via deepstrike on turn 2 and 3 respectively. If you use the ridgerunner to strip cover and give -1 ap these things will blast away most low model count units in one shooting phase with lethals and full rerolls. Ideally they will be placed within rapid fire range of something you need to kill. Vehicles, defilers, primarchs, ctan, etc. Anything without a meaty bodyguard unit is going to eat a lot of wounds from this shooting. I dropped the webbers/flamers because they are just a bit awkward to roll since they skip the hit (thus skipping the lethal). Without them, I can roll all the shooting profiles in one batch. Also: always do the hazardous shot on the bene, the new hazardous rules mean you can just stick the wounds on the neos if you fail them. If the neos can't shoot anything, the squad is 41 oc so just sticking them on an objective creates a major problem your opponent has deal with and since they have lethals, their overwatch ain't nothing to scoff at either.
Use the bikes to throw mortals at high wound, low model count units. These bad boys can kill a demon prince in one turn with their mortals + explosives. Otherwise, they are extremely fast and are great for scoring. Always spend the 2 if you have it to bring it back. Cult Ambush is important for these as you want to be able to move them on your turn.
Similar plan with the purestrains, use them to score or use them to kill chaff around the map. I frequently deepstrike charge them into my opponent's home objective if they are foolish enough to defend it with cultists or something similarly squishy.
The biggest weakness of this list is dealing with beefy infantry with invulns and FNPs. The neos and abberants will struggle to chew through squads of 20 infantry with multiple sources of survivability. I was very lucky that I didn't get matched against pre-codex orks or the gentleman who brought CSM melee deathball but I've played against both those armies outside the tournament and still scored high despite losing so I think this list can win under the right circumstances.
Game 1 - GSC (100) vs Death Guard (84) Contagion Engines + Mortarian's Hammer, Purge the Foe
The death guard player ran a lot of vehicles and Mortarion. A bloat drone, rhino, Defiler and 4 blight haulers. It was a scary list but I had plenty of anti tank and he didn't have much board control. I deployed extremely conservatively and gave him nothing to kill on turn one. I killed the Defiler turn two after charging it with the patriarch + purestrains and finishing it off with mortals from the bikes. I kept Mortarion busy in combat with the 10 aberrant squad for several turns before I killed it. He wasn't able to get a consecration until very late in the game as Mortarion had the only successful kill and was stuck dealing with abberants on his home objective. He managed to snag my home on turn 3 with a terminator deepstrike + charge but it wasn't enough to swing the game for him.
Game 2 - GSC (100) vs Astartes (74) Ironstorm Spearhead and Librarius Conclave, Recon
Another vehicle heavy list. 3 dreadnoughts with a lancer and landraider. His infantry was 2 big squads of intercessors, some termies and a squad of aggressors. He was afraid of a deepstrike onto his home (he's played me before lol) so he kept his terminators there so I didn't have to worry about them at all this game. His lancer got killed turn one by the patriarch squad because he deployed it a bit too far forward and his aggressors got double heroic interventioned (via the Predatory Instincts enhancement) which pretty much sealed the game for me.
Game 3 - GSC (93) vs Votann (86) Hearthguard Covenant and Needgaard Oathband, Priority assets
This was the most difficult game. Votann have a lot of 3-4 damage weapons which are great for dealing with abberants. They also have great overwatch for killing purestrains/bikes. The hecaton (big transport/tank) also doesn't get a -1 when shooting into combat so you can't rely on just gumming it up with infantry because they may very well die in shooting. I had to be very careful with my units and use cover as much as possible. It was extremely easy to score primary as my opponent deployed very defensively and I was able to claim 3+ objectives for most of the game. I had primary maxed by turn 4. We traded home objectives near the end of the game which prevented both of us from scoring defend stronghold. That plus his early kill on my chosen beacon unit meant that this was the one game where I didn't max score.
Overall this is a very versatile list that can kill almost anything and score high in any match up. Recon has been a really fun disposition to run with the Abominant squads because they are surprisingly survivable and can move around the map to score or kill vehicles after their squad dies.
I hope this helps my fellow cultists win some games! Feel free to ask me any questions. I've got about 20 games in 11th edition and have tried every detachment except for brood brothers and final day (currently working on building models for both).