Cheese fest ? - I wonder if a lot of players who use this term use this play style because they cannot beat the encounters without resorting to it. Because certain mechanics are available doesn’t mean you have to use them.
The game is very hard and frustrating for a normal game mode if you don't use backstab, highground advantages and jump. If you use them it become easy. If you use barrelmancy, poison, dipping, eating each turn, shove,... It become very easy.
You should try different playthrough and different mechanics limitation to understand how the game difficulty is(n't) balanced. Everything is made arround Larian's homebrew.
This is true. Larian expects you to use their special mechanics to win (imo-cheese). If you try to stick with 5e rules, you are handicapping yourself, especially since the monsters use Larian mechanics to the fullest.
Several of the "special mechanics" are 5e already, Coating your weapon with poison is the same mechanic as dipping your weapon in fire but gives the player more versatility to overcome damage reduction (if DR is even a thing anymore). Backstab is a buffed version of flanking, during a flank with two characters vs one they would be considered flatfooted and rogues would gain sneak attack.
Height advantage, explosive barrels, etc. sure you got me on that. There's a huge thread already on height advantage and barrelmancy has been nerfed already along with basic spell's doing additional floor effects.
There is actually a big problem with 5e rules where it is way to easy to break a spell that requires concentration. There is no concentration skill nor is feats as frequent as they used to be in 3.5 or pathfinder. Which both would buff the ability to stay focused on the spell while in battle. With the lackluster focus on concentration in the rules, those spells become more of a waste of time and unnecessary feature.