Note that I'm talking about ME1; I never played ME2/3, since I never cared about their combat systems (and it's not very interesting anyways)

Also note that in some fashions ME1 does succeed in its goals, and outdoes SC2, but it doesn't really matter for this; whats interesting is what it failed to learn from its 20 year predecessor

tldr; SC2 does less work and gets much further with it, both mechanically and narratively, in a lot of ways that ME should have learned from. Instead, it seems more like they weren't even aware space operas existed outside of film.

the same mistakes include things like the landing/mining minigame, which is terrible in both games in similar fashion: it's kind of interesting for the first couple rounds until you realize its repetitive, mechanically simple and somewhat poorly controlled. In ME, its a tiny-bit alleviated by being in 3D, so driving around is more fun, but the geology remains dull and generally pointless.

Both feature the nuisance of having to iterate over every world, looking at the stats, and dropping it for 80% as its unlikely to be worth exploring. SC2 also acknowledges its mediocrity and generally avoids requiring it, with very few quests requiring it (and the coordinates usually given), and it becomes mostly unnecessary for mining further into the game as you find massive resource deposits. ME's 3D becomes a negative in this regard, as they still use it for questing throughout the game, and while SC2 has a small 2D screen to parse for the quest location, ME requires "exploring" the dead lands to find whatever marker. The lack of variety becomes more obvious due to the 3D environment (and the amount of time you're stuck in it) compared to SC2.

Notably, the existence of shit exploration in SC2, and how memorable it ends up being, should have been a strong indicator to ME not to pull the same shit

Probably much more based on personal preference, but the much more open, broad and simple narrative of SC2 is more compelling than ME's, at least partially because there's less for them to do a poor job in. ME has bigger and more varied politics between the races, and by far is the more serious game, but this also leads to a lot more stupid politics. Trying to give a bigger background to the races just leads to each race feeling not much different from humans, because, well, they basically are humans in a different skin. Same politics, same motives, different colors. SC2 "cheats" by simplifying races to really distinct, bare-bone traits, and going from there.

The spathi are absurdist cowards, and that's all they are. Their background stories, and all their operations, derive almost entirely from their extreme fear of everything. The Zoq-Fot-Pik are a strange, friendly symbiotic trio of aliens, operating by weird language rules, and odder political preferences. A lot of races lack a clear background, told only in minor hints, and this makes them feel more alien than ME could hope to accomplish. Compounded with the lack of requiring them to walk around and such, primarily being differentiated by their voices and tiny gif-like animations, leaving it to the imagination.

ME makes the mistake that SC2 didn't: aliens are interesting by their very nature; the haze of information is what fuels it. SC2 also has the benefit of being comedic, so it can come up with less plausible stories. ie from SC2's wiki: "For another fifty thousand years, the Zoq, the Fot and the Pik relaxed in the forests, until one day one of each race was walking up a steep path looking for something to eat, when a bolt of lightning struck nearby. The bolt of energy carved a wheel-shaped chunk of granite out of a cliff. As the rock began to roll down the hill, some dry grass got caught in its hole, and since the rock was still hot the grass caught on fire. Thus the Zoq, Fot, and Pik simultaneously discovered the wheel, fire, and religion"

But regardless, it puts out the feeling of Star Trek far better than ME could ever hope to accomplish, at least partially because the game doesn't try as hard. It might be argued that ME was leaning more towards Star Wars (space drama), but even then, it fails by virtue of expunging too much information (and not being grand enough)

And there are other smaller things like SC2 granting a greater degree of freedom, again by virtue of doing less work on its weaker parts. The combat is simpler, and doesn't require it for the most part, and is generally better off for it (ME loves to pretend it has a good enough system; it does not.) SC2 still has the fault of having an annoying amount of combat for what it is, but still far less so than ME (and doesn't really require it for any narrative events). Exploration is more interesting in SC2, again because of the freedom, narrative structure, and it wasting less time on its weaker components.

And of course there are those mistakes that just come out of modern game design, but these are unsurprising:

fast travel: kills any sense of distance and exploration (in ME's case, all travel is fast-travel, so space doesn't feel at all ... distant. The citadel feels larger than space.)

quest-logs: kills any sense of autonomy in the narrative, and background-discovery, and even exploration itself

emphasis on combat missions: ME does not have a good combat system. There's absolutely no reason for it to emphasizing it.

emphasis on player character: It's a space opera, and its focused on upgrading your player character & co? This is just...wrong. Personal preference, but focusing on ship upgrades is much more sensible. But then, the game barely involves space in the first place, so maybe not. It could have been a medieval fantasy and it wouldn't have been too different, in a lot of regards. (ofc, it is bioware, so maybe it really did derive from medieval fantasy)

dialogue wheel: adding morality to the choices, and then making it obvious to the player? It's just absurd. narrative branching on action alone is sufficient, less work, and obviously more correct.

combat-wise there's a lot of mistakes as well, but they're uninteresting for this discussion, and have a lot of other predecessors they failed to learn from. Suffice to say, its not a shitshow, but its not well done. Whats more interesting is that the designers failed to acknowledge its lack of quality, and account for it (or maybe they did so intentionally; its an EA title, so publishers are likely a good deal at fault for that one).