I started DMing on 2018, with DnD 5e. I spent around 5 years playing as a Dungeon Master for my friends and was able to run combats of up to level 17.
There are many problems with combat in DnD 5e. Most of them come from how bland the system is.
First I want to talk about mobility. Whenever you are fighting 3 enemies in dnd, if you are already at melee with one of them there is literally no reason to step back and reposition unless there has been an enviromental point of interest that gives you a significant advantage on combat. If you want to run towards a different enemy before defeating the one next to you, you will take an opportunity attack. And if you spend your action to Disengage and avoid it, that same enemy can just follow you and keep being in melee. This leads to combats between melee creatures just being big blobs of creatures all next to each other.
Not once have I seen a combat end up any other way. Just all creatures close to each other with mages and archers around casting spells and shooting in place. Once you are "in place" there is no reason to move. Imagine this in an action videogame: You are fighting a boss, you are not moving at all and just spamming the attack action. That's basically what DnD is. Whenever you kill your opponent you move to the next one and repeat. There is literally no point of moving once you are engaged in combat. Trying to go for the mages is useless because you will take an opportunity attack, attack the mage, the mage will teleport even further back and your previous combatant will catch up to you and attack you.
The lack of inherent taunting mechanics is also a big issue that keeps classes like the Paladin or the Barbarian unable to provide their roles in combat. Why would a boss attack the guy with twice the HP of the Wizard, half the damage and 5 extra points of Armor??? Even if the boss tries to always move around chasing the mages, its just leads to running in fucking circles.
Damage types barely matter outside of "oh no, fireball doesnt work on the boss, well, thankfully I took lightning bolt that deals the same amount of single target damage". Resistances to physical damage only makes martials more useless, since they barely get access to any damage type outside of the three physical ones outside of magic items that most people don't give you any flexibility unless you have multiple different magical weapons with different damage types.
The amount of videos trying to "fix" combat in DnD is absurd at this point. The day I tried Magic the Gathering and saw how synergies made combat so much more interesting just by having a monster that says "R: This creature gets +1/+0 until end of turn". One ability, that's all that you need to make a creature so much more interesting. Triggers, keywords and shit like spending resources to activate effects makes combat so much more strategic without needing to ever study medieval war tactics.
I learnt more about making interesting combat challenges playing MtG for a WEEK than I did on five years of playing DnD while actively trying to make combat more interesting again and again. And what's even worst is that both games are owned by Wizards of the Coast, you'd think they would share some of that design philosophy.
TLDR: Combat in DnD is bland and you'd do better using MtG design to make monsters.
