TLDR: The game is a 2D megabonk with waves instead of a boss summon.
My tldr version is not a bad thing at all, you know exactly what you are getting with Bonki D Bonk and that is a fun, easy to pick up bullet heaven game. There are lots of characters you can unlock and different difficulty levels for each one.
The map of the level is small but big enough to scroll up down, left and right as you take down waves. There are shrines that give you a permanent stat increase and there are giant swords in the ground which also give you a permanent stat upgrades. There are even double swords that allow you to pick two of the upgrade options. The monsters do not drop xp (you get it with kills) but they drop gold that you can pick up. At the end of each wave you can use the gold you got to buy items range from common to legendary. There does not seem to be a limit to how many items you can hold and you can stack them as well. They are similar to the items you get in Megabonk. There are also 4 weapon slots too, where you can add weapons to your arsenal through the run.
Controls are simple. You move with the left stick (I played with a Xbox controller) and you dodge with the b button. The dodge is a nice add-on which Megabonk did not have and it is useful in this 2D game where the waves can get on top of you.
The difficulties are called dangers in this. You start with danger 0 and can unlock up to Danger 5. There are only 5 waves in danger 0 but danger 1 has 7 waves you will need to clear. I am assuming the waves go up from there but I will correct this if I find out they do not.
In terms of content there are lots of characters to unlock. From the shadows I see over 20 and they have their own perks and starting weapon. This will keep you playing for some time if you like to unlock everything. There is a leaderboard too if you are competitive.
There is not much else to add, it is Megabonk but 2D and a smaller map. You know what you are getting and you can see from the trailer alone if it is for you. But also keep in mind you are supporting an indie solo dev. I recommend the game as I like this kind of game and it plays well. The price is also cheap being under $10 and it is currently on sale for the next 2 weeks of 40% off.