I'm going to have to respectfully disagree with you on that.

While ME1 might objectively have more different types of enemies, there's a lot of redundancy. Fighting humanoids that use rifles / rockets / snipers is no different than fighting geth that use the same weapons, only difference is that geth have more shields. A krogan charging at you is 80% the same as a geth devastator charging at you.

for "non humanoid aliens", all I can think of are rachni and varren, and those only appear like twice or thrice the whole game. Even still, you don't change strategies while fighting them, just keep a distance and shoot. And the bosses weren't really special. The geth armature was just a normal enemy that you fight on foot instead of in the mako, the thorian just had you fight husks, the noveria boss has you fight asari adds, and Saren imitated the geth hoppers, just with more damage.

The only enemies that really required strategy in ME1 were probably geth primes and charging krogan warlords, who will fuck you up if you're not prepared with the right skills. Everything else was just point and shoot.

In ME3 however, each "faction" had unique units, and barring the foot soldiers, they all needed a different playstyle to take down, and your team composition helps a ton.

For example, you have the Cerberus dudes with shields: you can't just shoot them normally. You need to biotically pull their shield, or have armor penetrating weapon mods. If you have neither, then you need to either flank them from behind, or try to shoot through the little mail slot (which is pretty risky).

The Engineers and their turrets will kill you in a fraction of a second on higher difficulties. You need to have someone with sabotage to take over the turret, or have a team comp with tech bursts to take down the turret ASAP, otherwise it'll keep you supressed and hiding in cover while enemies flank you. Then you need to take down the engineer before he pops more turrets...but he has a habit of hiding behind corners, protected by other troops.

You have like 4-5 unique enemy types like this per faction (barring the boring grunts), and if you don't have a balanced party, you'll have some difficulty. Enemies that could be taken down in 3 seconds might take 30 seconds instead, and that buys time for other enemies to get the advantage on you.

Then you have different enemies synergizing with each other. Husks on the reaper faction exist purely to get you out of cover when you're in melee... and that'll make it easier for ravagers to 2 shot you. If you're too occupied with the cerberus shield dudes, you're going to be flanked by other troops eventually. Phantoms, brutes, and banshees also force you out of cover and force you to be mobile, unless you like getting sync killed, and they require a concentrated effort to take down (plus, when they make you mobile, you're again an easy target for ranged enemies).

...Then in the multiplayer / DLC arena, you have the collectors. They have swarms which put all your abilities on cooldown, so my vanguard who relied heavily on biotic charge to restore shields had to be way more cautious. Scions hit HARD and fire a lot. Praetorians have the health of an atlas / banshee....but they fly around you to flank you and now you're surrounded by enemies on 2-3 sides.

Also, IIRC, ME3 objectively has more abilities than ME1, and each ability gives you choices how you want to evolve it (ie Nova for vanguards can do half damage but drain less of your shields, or you can get increased ranged on it; passive skills let you pick whether you want more health or melee damage; overload can be an aoe with decent damage or single target with a ton of damage, and so on).

And then you have team composition with ability combos. If I get Garrus and Tali on my team, they can do a tech burst explosion with overload + energy drain, which can take down an atlas's shields in about 2 hits, or strip a group of enemies from barriers / shields. However, this comp will have trouble with armored enemies, but I made up for it with vanguard Shep with incendiary rounds + biotic charge to "detonate" the flaming enemies for a combo, or I can do a biotic explosion with Reave + charge/nova.