It explains why people live on planets not O'Neil cylinders.
You know, people might end up just
choosing to live on planets rather than in O'Neil cylinders (which the Citadel actually looks somewhat similar to while the arms are partially open, but I digress). It isn't all about what's most efficient. I know I'd rather live on a terraformed Mars than an O'Neil cylinder orbiting around Mars, resources be damned.
It doesn't explain magical brain cancer but you can't have everything.
Biotics are definitely the softest thing in ME, but it does kind of explain them. The mass effect is all about using Element Zero to alter the mass of matter, regardless of how. Biotics are people who can do so consciously by using mutated nerve nodes that have element zero inside of them. That's pretty far-fetched, but people in ME can't use biotics to any useful degree without mechanical amplification, so that's a little more plausible. The body, for whatever reason, might just decide that element zero is supposed to go in the nervous system like iodine is supposed to go in the thyroid.
Almost all of the biotic powers make sense in the context of what element zero does, except for the weird vampire ones that ME2 had. Pull and Push reduce their mass significantly and their existing momentum makes them take flight once that happens. Singularity is self-describing. Barriers rapidly reduce the mass of incoming projectiles, and so they just bounce off harmlessly. Charge is effectively a short-range personal FTL jump.