Many people are complaining about the changes to combat to make surfaces and explosions much more prevalent and such, bonus actions, disengaging... Changes are not inherently bad, people aren’t complaining about changes merely because they hate the idea of change. Changes DO need to work together.
If your Mini loses a wheel and the garage says "I didn't have any wheels for your size, so I put on one from an 18-wheeler", your car is not going to drive as smoothly as it's supposed to.
The crux of the issue is that some systems remain based on 5e – armor, base chance to hit, hit points – which have not been altered to match Larian's other changes. Damage
Cantrips creating surfaces and status effects which don’t have any saving throw to avoid the damage. Being higher than a target gives advantage, which statistically averages out to a +4 to rolls to hit (and lower is a -4 on rolls to hit)
. That makes high ground the single most important thing you can have in combat, and it adds artificial difficulty when the enemy starts out on the high ground (*cough* The harpy fight). There are often explosives everywhere, enemies who have lots of fire arrows and bottles of alchemists fire.
Hit points and armor do not take those into account. There are many additional sources of damage - many without saving throws to mitigate any of it, and for anyone on the low ground, it is far easier to be hit and far harder to hit, which means you take more damage, and deal less damage, which drags fights out longer.
The AI is still reminiscent of that from DOS's AI 2.0, where it ignored heavily armored targets to go after squishy mages. In DOS 2, that worked out fine, because of cooldowns instead of spell slots for healing and mobility spells, and bedrolls for free out of combat healing - you could always start a fight in DOS 2 at full strength. D&D is based on attrition - managing resources.Healing
The healing economy is similar to that in 5e - healing cannot keep up with the amount of damage you take, but there is now more of it because of surfaces and reduced AC meaning you take more hits. BG 3 currently tries to balance out all the extra damage you take by adding in food which now heals, and having long rests be unlimited.
Food was pretty useless in DOS 2 because healing was cheap in that game, but leaving aside long rests, healing is very expensive in this game. That makes food super-useful - as good as or better than that you get from potions and healing spells, which consume precious spell slots. And you can eat food as a bonus action, which makes using a spell slot for healing anyone who is not dead feel even more like a waste.Rests
The long and short rests also need balancing. You're only allowed one short rest between long rests, which are unlimited. The balance between Warlocks and other casters is based on multiple short rests between long rests. Allowing unlimited long rests makes slot-using casters far more stronger, as they can be at full strength for every battle, and makes Warlocks very underpowered. I think 3 short rests between long rests is the right number. It should be no less than two, at least.
Long rests being unlimited is exactly how it worked in DOS 2, which was based around going into every encounter at full strength. If there are any additional consequences, they are not clear at this time.
I don't know how to limit long rests. The best idea I heard was to have some sort of hidden fatigue meter, which increases based on the average number of steps the party has taken since the last long rest, and also increases when receiving any sort of healing. You would not be allowed to long rest until a certain amount has been passed.
This is better than a method based around consuming resources to long rest, because it prevents you from being blocked for lack of resources - you can walk around some more. This is better than a random encounter system because if you're really on your last legs and NEED the long rest, you won't be attacked at your weakest. (Or you won't need to long rest immediately after a long rest because the random encounter went badly for you.)Conclusion
The changes to cantrips led to the changes to give enemies more ways to make surfaces. When combined with the changes to allow height advantage and disadvantage, the issues with taking more damage with the same amount of armor and hit points are clear. That led to the changes to allow for cheap healing through food items and to unlimited long rests, which leads to a more D:OS-like "fully charged for every fight" combat style, which leads to far more powerful spellcasters (and weaker warlocks), which will lead further balance problems down the road.
The changes have a domino effect, which will lead to a power arms race. This is why some of these changes should be walked back closer to the standard 5e gameplay. Not because changes are bad, but because the current balance is fighting against itself - there are two competing systems with different design philosophies at play: The attrition-based D&D, and the full refresh-based D:OS 2.
These are just some thoughts I have about the balance between damage and healing.