For Stadia I can confirm Combine does not work when you are in controller mode. However, when you are in desktop mode it does open up the combine menu, but it's very flimsy and doesn't work very effectively. I tried combining several different ingredients into the booze bin at the goblin camp, but only the poison that you coat your weapon with would combine. Sleeping potions and Hag's potions didn't combine.
Other things I've noticed in regards to Stadia when using a controller:
1. I've seen videos of other platforms where you can see the field of view from enemy's and when the red lighting disappears you can go into stealth mode. For me on Stadia I have to spam the stealth button hoping it finally works before I can see if i am or am not in field of view.
2. Sorting doesn't work at all in the inventory screen
3. When you're moving the cursor around on inventory screen or equipment screen you click over once and sometimes it goes 2-3 spots over, doesnt' register at all, and/or is extremely slow. If you could change it from a cube by cube slide and had it more like a moving hand or cursor it would make menu navigation way easier.
4. Desktop view for BG3 especially in combat is vastly superior to Controller view. It would be amazing if we could manually select which view we would like. I actually prefer using the controller when I'm on anything other than my Desktop (chromecast / browser on laptop / etc).
5. There are dozens of times in a playthrough when you're in combat and you're trying to cancel out of the action by hitting the B button and it will end your whole turn. Normally you have to hold B and you'll see the little timer, but almost every battle it just insta skips. Great example is if you're trying to see if you have enough range for a specific spell, realize you don't so you hit B, then it exits out and skips your turn instead of choosing a new action. Perhaps the end turn button should be different than the cancel button?
6. Not sure if this is a controller issue or game issue, but hitting the L1 and RI buttons to cycle through items, people, objects, etc. doesn't work effectively. You constantly have to move your character around to get in just the right angle for it to finally connect. Example is when you're escaping the ship at the start and you're looking at the computer screen with the 3 sigils and the 3 buttons to either kill or passify the guys on the tables. I could cycle through a couple buttons and sigils but had to spend 2-3 minutes moving around to get them all to click.
Hope this helps! Even with a lot of the bugs and issues it's very fun!