It really depends on what you mean by “works well” and “upgrade”.
From a casual standpoint, it’s a great game if all you care about is getting probably 1 copy of your favorite character, or if you want a character collector. The production values aren’t bad, and the rates are decent. To top it off, it’s a lot less complex than Epic 7.
I’m not sure what your main focus is since you want to have a casual/not tryhard game yet want to know if you can make your favorites OP, but I’ll try to break it down.
In terms of upgrades, characters gain stat bonuses from merging duplicate copies, which is very difficult if you character is 5* exclusive, but much more doable for 3 or 4 star units. By building characters, there are a lot of budget skills that will serve well, but if you’re interested in more premium or varied skills (i.e. Distant Counter being the most prevalent and well known since launch), then it will usually be locked again to 5* Exclusive characters. If your favorite is a 3/4 star, then it’s very possible to fully merge them to +10 (you need 10 copies to max their potential).
In terms of working well, most characters with a decent kit and strategy can clear PvE content, and there is a large amount of F2P guides out there to aid players clear the content. However, top tier competitive PvP will often pit you against premium units and skills.
I used to enjoy the game a lot, but my biggest criticism is that character progression in the game is tied to summoning at least 90% of the time if you want to upgrade your favorites. Merging will typically require summons, skills require summons, and another resource (dragonflowers) that add stats to units require summoning. If your goal is to collect a few of your favorites and hear their voices it’s totally great, but if you want to do anything seriously it’s going to be a long time of saving currency, strategically rationing your resources, and then summoning selectively for premium skills and copies for a select few of your favorites.