Pineapple's video has made me interested in the game, but I don't really have experience with the franchise either. Is it difficult in the sense that you need to be adapting and perfecting your mech's build, or difficult in that no amount of overpowered robot parts will give you an edge compared to Gitting Gud?
I see people rave about how challenging the series is, but the same thing was done for Dark Souls.
A little bit of both, I don't want to insult your intelligence but I'll spell it out anyway for clarification; parts are different, and can bring different pros and cons to your mech. Lightweight parts are generally speedier, but lack defense/health; heavier parts are of course the inverse. Now apply this to your leg units, which have a Max Weight and you need to balance everything around there (game depending, you can sometimes go over, but shit gets hard). There's also weapons, where reloads/refreshers were never a thing; what you bring into a mission is what you're expected to finish the mission in... and some missions can and will throw curve balls at you.
One mission in the original AC that's memorable is there's an underground base, and I forget what all you need to do, but you need to go down there and do something. The problem being, someone released some sort of gas that eats metal, so every milisecond you're in the base, your armor is going down like you're poisoned in a rpg... oh, and base defenses are active and don't like you. Do you go for speed? Armor? Something inbetween? Oh, and don't forget weapons, since you'll be in an underground tunnel system, weapons that arc (like vertical missiles) are completely useless. You want energy weapons? Make sure your generator won't burn out, you also need the energy to boost to be fast, otherwise the gas will get you. No one bothered to tell you where you mission objective is either, so you're gonna have to go down there and find it yourself.
And while this doesn't happen in that mission; there are a number of missions where you think it's over or you're getting near the end, only to have your operator tell you something like "Incoming AC. AC Unit identified as Nineball." Oh, and Nineball is the #1 Ranked Raven... and he's in your way... and it's a vertical fucking shaft with nowhere to hide... then you get past him, you can run but let's just say you shot him down... ... ... then there's another Nineball... and you start questioning your decisions in life. How's your armor and ammo holding up? Oh yeah, and failing this mission doesn't kick you back to the menu or anything else, it's a Game Over.