There is a slight difference in context between both games that makes the criticism much heavier for one instead of the other.
Solasta's potential cheese is based more on the encounter design and the tools available to your team. Wall of Fire and Spirit Guardians are crazy, but they are legitimate spells that you can't get until you're about halfway through the game.
Your example of spitting spiders in Solasta is slightly disingenuous. The early level spiders only possess melee attacks, and I didn't run into any spitting spiders in Solasta in the EA content that was stated to run for half of the game (I am personally waiting until I am done with WotR beta phase 2 to restart Solasta from the beginning). I think by the time I'd run into them, I'd have a bunch of fireballs available to burn them with. The BG3 phase spiders have AoE spits, which is more than a bit nuts for an early game encounter where you have very few ranged options and pushes any fight involving them into what I'd call 'trap clairvoyance design'. Granted, I wasn't too miffed because I am already used to DOS2-style encounter design, but it would feel insane to anyone else.
Wall of fire/Spirit Guardians is OP ONLY because the AI and encounters in Solasta are not strong. An intelligent monster shouldn't walk into a wall of fire unless they are immune to fire. They would look for another option, or move to ranged strikes. Yet I have never seen the AI do anything particularly smart.
On top of that, I am level 10 and in all the encounters I have yet to have anyone use a powerful spell on me. Every single time its a low level Magic missile or a Cantrip at most. Not once has anyone thrown a fireball or lightening bolt at me or my group.
Compared to Bg3 and enemies WILL use 1st and 2nd level spells at you all the time. Sleep and magic missile are common but I have seen Flaming Spheres, Thunderwave, Bane, Bless and a host of other decent 1st and 2nd level spells. Enemies will try to drop stuff on you, throw things at you (flasks and grease), use the environment, shove you off cliffs, knock out concentration, focus down high value targets, or casters, they seem to know about advantage and will go after anyone who has their back turned and they will try to get height advantage on you. They are jerks. I love it.
Solasta is a good game to learn about a limit range of D&D combat rules. There is also a lot of detail oriented micro-management (optional). Its not a challenging game by any measure even on the highest difficulty settings. And spending the time to make sure you are organized and have everything you need never pays off with encounters that really test your preparedness. I ended up with a stack of revive and raise dead scrolls I never used because no one ever died on my side...or got knocked out.
