The hardest part or at least the one that gave me a problem was entering the building on Noveria and facing 3 Krogan and the Juggernaut
I find ME1 and ME3 easy on insanity mode. ME2 is a lot tougher.
I agree on the verdict that, in general, ME2, is the tougher game of the lot. There's one sidemission (?) in ME2 in particular which (on insanity, as always) I don't remember clearly to have even succeeded in yet . You have to take down three of four Atlas mechs before they destroy some crates or something. Suspect I've had quite unsuitable gear and companions along for it. If I did manage to pull through eventually, it took a lot of attempts and me thinking "How the h*** are you supposed to pull this one off?". 
I like risk. If I didn't I wouldn't have played on Insanity. But I prefer for that risk to come from different factors than artificially prolonged fights. It is not engaging to me to fight one enemy for two minutes repeating the same moves until it dies. I don't feel threatened in such encounters. I want to be forced to change tactics, adapt. ME1 enemies very rarely force me to adapt. ME3 has more of such situations. ME2 has the best balance IMO.
I agree on risk from different factors is preferable to risk from merely long fights. I also agree on it being less engaging to repeat the same moves in order to eliminate the opposition instead of having to employ different tactics. So in this respect ME2 is the better game for sure. No arguing there.
But I find long fights vs enemies with superior protection and firepower, as in some sidemissions early on in ME1, definitely challenging. They tend to force me to use the same tactics, popping out of cover to shoot before they shoot me, otherwise I'm usually done. That's basically how real combat in modern warfare for infantry is carried out. Pretty repetitive. Flanking moves etc are also carried out here, of course, but it usually also involves higher risk (leaving cover for a moment etc), so should not be employed unless necessary for winning the fight (quickly enough). As for the fights in ME1 being "artificially" long, that would depend on the alien weapons and technology used in this fictional world and I don't know enough about them to say so. If we met a hostile alien race with superior protection and firepower but bad tactics in real life, my guess is that we would still find the enemy difficult to deal with. We would not be able to outgun them in long and simple duels. But for pure gaming purposes, where the line between challenge/enjoyment and tediousness/boredom goes under similar circumstances in ME1, that's just a subjective matter. Your opinion is as good as mine.