This is kind of a stylistic thing, DA: O has way deeper combat than both ME2 and 3, but it's a true cRPG system and it takes a bit of research and work to fully understand it and use it. A lot more tactical.
ME2 and 3 are straight up shooters, and most people would rather shoot shoot shoot instead of reading walls upon walls of text to find out what abilities they should get and figuring out what gear they should have.
Generally speaking these were my thoughts in my final playthrough of Origins back in 2018:
I've already beat Origins like 3 or 4 times on the Xbox 360 years ago so most of this will be about DLC that I didn't play like Awakening, Witch Hunt, Leliana's Song & Golems of Amgarrak.
One thing I do notice after playing Baldur's Gate and quickly jumping back into Origins is how "tactics" work. There is less use of positioning in Origins since you can't necesarilly pick the formations of your party like you can in BG. Since you have many fights in the game that start right after you enter dialogue, you party gets teleported to where dialogue/cutscene happens and then a fight breaks out right in front of the enemy, many times leaving your party members with lower constitution or lower stats at a disadvantage.
Nightmare is kinda easy in the base game especially when you have mages. In the beginning there is a challenge and if you fight enemies you shouldn't for your level, they will spank you, but other than that, it's far too easy. The game just buffs the hell out of the health of enemies but it is still too easy. I also started using mana clash this playthrough, something I didn't in previous playthroughs and it was outright killing the more powerful mages, demons and mana users.
The first Cauthrien fight is pretty much impossible on Nightmare. She killed everyone in my party in one or two hits each and can resist or is immune to every attack.
I remember when I played this portion on Normal years ago, it was a tough battle but manageable, here the health buff is so ridiculous that it's near impossible to take her down.
It is admittedly worse, that if you do fight her the 2nd time, she is not this overpowered.
DLC is easy on Nightmare too until you fight The Harvester in Golems of Amgarrak. That fight just sucks ass, the Harvester is slow but hits hard as hell and somehow your weapons constantly miss it. He will spawn more helpers by doing an AOE attack that can take out nearly half of all your allies health in one hit. You are also given some of the worst allies in the entire game. One Golem, one rogue, and one warrior but you are not allowed to respec or choose any of their skills because they're fully leveled up.
These minions that he summons can outright one shot I and my companions on nightmare. The boss stops taking damage when the screen turns red and you have to press a switch to make him take damage again.
The entire fight is quite boring with the main strategy I used being constant healing and kiting with my main character mage. I can't fathom how this fight would've turned out if I wasn't playing a mage.
Worst of all there is a 2nd phase for this boss fight. The boss is smaller and moves around more, and now you can't turn off the switch, it turns off on its own after a time. Now more mobs consantly respawn and they're even faster before and do enough damage to kill you & your allies in two hits.
Awakening is the best DLC, I actually quite like it and feel that it has far less fluff than the main game and Amaranthine is quite a cool city to explore compared to the rest of the game.
Awakening introduces a bunch of talents for the main classes and new specializations. The Rogue Legionnaire is Overpowered as hell, being able to ignore all magical attacks and take no damage at all to your health is OP as fukk.
I used a few mods (credit to lilura):
Advanced tactics
Extra dog slot
Make console commands visible
DA2 Mabari
DA2 Qunari
Faster Combat
GT Core Rules Pack
Back when I played origins years ago, the only mod I used we're armor mods and a mod that allowed you to break chests because I didn't want to constantly carry a rogue around.
I also ran into an annoying bug in this Ultimate Edition that I didn't before. A memory leak bug that made the game keep crashing which is annoying as hell.
DA:O was an enjoyable experience in total. The middle really is a slog. I don't like the Fade anymore, I don't like the dungeon crawling in the Dwarven Thaigs, they look far too similar. I probably won't be playing the game again anytime soon, but I have played it a lot so that's good enough.
Playthrough over a span of two months.
DA: Origins - 50 hours
DA: Awakening - 14:13:19
Leliana's Song - 1:21:58
Witch Hunt - 1:41:50
Golems of Amgarrak - 1:35:53
make sure you spend x hours collecting enough materials or party members will die in a scripted event leading into the last scenario btw. can't forget the riveting walking simulator loyalty mission for the fish breh or the daddy issues in space episode. last mission was great but lets just literally never talk about how dumb the collectors are
...but they been dumbed downed doo doo shyt ever since. We'll see what they do with Starfield buuuuut.... 
