For context, I've only played about 4 hours and the furthest I've gotten is the exit door on the Procedural Factory, but it's been enough for me to realize that if this game were only singleplayer or if I didn't have a friend I was excited to play with for the full release, I would give up on the game really quickly. I enjoy the basic gameplay twin-stick shooter action and I love tower defense games, so this feels right up my alley and seems like it would be incredible in multiplayer. However, the AI companion is very lackluster from a tactics and cooperation perspective and tends to feel like more of a hinderance than a help. The way I finally beat the first floor was literally hiding Bunker in a corner while Blaze defended the crystal against all the waves alone, because she would die if she was left anywhere near the action.

So, my complaints with the AI companions:

1. In multiplayer, a friend could take one group of barrels while you take another. In singleplayer, it would help if an AI following you would break barrels in the current room. Breaking the barrels is boring and time consuming (bad in a game with real time waves) but necessary for the resources. With two characters, you could get through it at least a bit faster. It's especially painful when using Blaze and having to wait for his slow rate of fire.
2. In multiplayer, you could tell your friend "come get this weapon" or "go buy a medpack" and then continue breaking barrels or upgrading/building turrets in another room while they move. In singleplayer, it would help if you could tag a weapon or shop item for the AI to come get, instead of manually moving them yourself or needing to stay in the room where you want them to go.
3. In multiplayer, a friend could be checking which turrets near them need upgrading, while you check the ones in your area. Instead, you waste half the time between waves running between turrets.
4. In multiplayer, a friend could fall back safely as they lose ground (or chase a few enemies that got past them and are moving to attack the crystal). I realize that level of strategy is difficult to implement for an AI without taking away player control. At the very least, there should be a "teleport to crystal" command you can give the AI in the middle of a fight instead of leaving the other character vulnerable when you switch to the AI to make them fall back. This is especially an issue when you are trying to hold two chokepoints far enough away from one another that you can't see the map to know if they are being overrun (or completely ignored), only the health of the AI and the crystal.
5. In multiplayer, you and your friend could cover different fronts (ie. you go ahead of the crystal to clear the way and I'll go behind to hold off those following us). The AI only has "follow", so the only way to achieve the same effect is to stand as close to the crystal as possible and run both of you between the two fronts. I'm not sure how this could be fixed without making controlling the AI too complicated (maybe a few set "formation" commands?) but it's definitely the biggest dealbreaker for me. Without more control, you lose alot of the tactical element and can only rely on the pure firepower of the character you are controlling.

It's worth noting that some of these issues (1-3) could also be alleviated by removing the real time waves (something I'm also in favor of), since it would make it less risky to take the time to move the AI companion/break the barrels/upgrade the turrets and just... kinda boring. Complaints 4-5 might be more difficult to work around but are the biggest detriment to my enjoyment, since they make the AI companion just feel like a turret that can die. And will die. Alot. I've even tried playing where I switch back and forth between characters frequently to keep them from standing in the middle of a swarm, but that just made both of them die faster.