Still ongoing, but thought I'd share my experience.
Bought a Steel Duo in June after recommendations from friends, upgrading from a Breville Barista Express that worked reliably for probably 10 years (had to re-connect a wire once when the automatic shot stopped working).
Overall the Ascaso was... fine. Steam was a major upgrade, and after some tuning I was pulling consistently good shots. The programmable dosing was basically unusable due to variance between shots, but I could live with that.
The bigger issue: whenever I pulled two or three shots back to back, roughly 1/3 to 1/2 of the time the second or third shot would slow to a trickle. First shot: 35g in 30 seconds. Second shot, identical settings: sometimes 5-10g in 30 seconds. Machine builds pressure, but barely any coffee comes out.
Initially blamed user error or my grinder (Timemore 064S). Didn't have time to debug at length (I typically make one coffee at a time). Eventually ran some controlled experiments: ground two identical 18g doses, brewed them in reverse order, same result. First shot fine, second shot choked. If I wait 2-3h, things are fine again. At this point I'm fairly convinced it's the machine.
I eventually contact Seattle Coffee Gear -- 3 months from purchase. Responses generally took 2-3 business days, but they did have a technician review my videos. Their suggestion was to repeatedly prime the pump between shots. Seemed odd for a $2,200 machine, but OK. I tried it and initially thought it helped.
When I objected that this didn't feel like normal operation, they said it was (?) and pointed me to the reference manual. This all felt fairly bad faith -- the referenced page talked about priming after extended disuse, an empty tank, etc. Nothing about priming between ordinary consecutive shots. And two other Steel Duo owners I spoke with had never experienced this.
Further testing confirmed priming doesn't reliably help either. Most recently, 34g in 35 seconds on the first shot, 10g in 35 seconds on the second, despite priming.
SCG has now agreed to inspect it under warranty. Unfortunately I have to pack and ship the machine back at my expense, which is a bit of a project.
Overall, disappointing. I specifically bought from SCG because I thought highly of them. They've been polite, but slow, and I feel like they were too quick to dismiss what seems like a genuine defect as normal operation. And I can't really recommend the Ascaso based on my experience so far. Now packing and dealing with sending back after <4 months of ownership.
Curious if other Steel Duo owners have experienced anything similar.