Collections are great, but I think they need some additional features to be easier to manage.
First of all, when I'm browsing itch.io from my phone, the buttons to add to collection, follow, etc. aren't visible at all. So, if I want to add a game to a collection, I have to leave its page to bring up search, search for it, and double-tap on it to bring up the add to collection button (if I just tap it, I get directed to its page again). This is extremely inconvenient. Compare to GameJolt where there's a '+' icon you can tap to bring up a menu to either add it to one of your collections or remove it from a collection it's already in. That also makes it easy to move a game from one collection to another, which brings me to my next point.
If you have multiple collections with many games each (I have one with 150+ games and one with (probably) 250+ games), managing games is extremely awkward. If I want to move a game from one collection to another, I have to add it to one collection and then scroll through a miles long vertical list of dynamically loaded games to remove it from another. This process would be much simpler if I could display my collections the way games are displayed on the front page, with a menu similar to GameJolt's '+' menu for each game. If so, I could just open the collection I want to move it from, add it to my other collection, and finally remove it from the collection I'm looking at. To make things a bit simpler, it doesn't have to be removed from the collection instantly to re-align games in the grid.