(Quite a lot of spoilers below)
There are at least two separate points here, which I think should be addressed individually.
1) Is the combat a faithful, or at least acceptable, videogame interpretation of 5E D&D rules?
On this question, I don't know or much care, because I've never played D&D, although I know Larian have said they had to mess with some mechanics for reasons of practicality and gameplay balance.
2) Is the combat too unfair or otherwise hard in BG3?
You'd think so, to look at some of the streams of people blundering around the map and getting TPKed in every battle. But honestly, if you play the game as I believe it was intended - explore thoroughly, read books, chat with as many NPCs as possible, collect *and use* items (potions, scrolls, vials, barrels), sneak around new locations, scout for useful environmental features (haven't seen anyone drop a brazier on Gut's head or a stalactite on the Owlbear, topple a statue on top of Ragzlin or lure a Spectator to a Torchstalk before blowing it up), position your team well, do your best to surprise the enemy for the bonus attack - most fights magically become much easier. If you treat this like a fantasy Fortnite, you're going to get your ass handed you on a platter.
I don't know how much you've experimented with the Gnoll fight, but given how hard you've found it, I'm guessing you've never got the dialogue with Flind, which (if you make your rolls) makes it 10x easier. This suggests you've been approaching from the cave side, which never triggers that dialogue. Come at them from over or round the hill to the south instead.
Regardless, with elevation advantage, maybe a familiar and some buffs, it shouldn't be too challenging. It's doable at level 3 although I sometimes wait till 4, depending on how badly I want Flind's weapon.
Hardest fights for me in descending order:
Bulette (esp when combined with HHs & Filro)
Minotaurs
Kuo-tua
Spider Matriarch
Gith
Masks
Hag
Attack on Grove #2 (if you want to save everyone)
Bernard & the Armors
Duergar/Redcaps
The gnoll cave fight is several places further down the list (but I may be missing a trick on some of the above combats.)
