Realistic? Well, let's see:
- We have an element that emits a field to change the mass of objects when you subject it to electric current
- We have aliens that are apparently all based on DNA, as it occurs on earth
- We can reanimate people that were dead for 2 years
- We can build a device that we don't understand and that can disperse the essence of a person throughout the galaxy and use it to change every living thing into an organic-synthetic hybrif
I wouldn't call it realistic by a long shot. Does that make it bad? Not at all. I think it started out really well because it used one really unrealistic thing (eezo) and built the entire universe and it's technology around it. Unfortunately as it went on, this hallmark of the series was abandoned in quite a few ways and more and more unrealistic plot devices were introduced.
Also, low yield weapons? Let me quote an extract from the codex entry on dreadnoughts:
"An 800-meter mass accelerator is capable of accelerating one twenty-kilogram slug to a velocity of 4025 km/s (1.3% the speed of light) every two seconds. Each slug has the kinetic energy of about 38 kilotons of TNT, about two and a half times the energy released by the fission weapon that destroyed Hiroshima."
These ships can basically fire mid-sized nukes every 2 seconds. Their destructive power is so devastating that they are forbidden from firing on a target with a planet at it's back on the chance that they could miss and basically obliterate the planet by accident. And at this point we haven't even started to talk about Thanix cannons and reaper weapons yet.
Sorry, I love the ME universe but to say that it's realistic or toned down or anything like that, I can't see that at all.
Also, most of the other franchises mentioned here, like Babylon 5 (soul hunters, telepathy and transformation from humans to energy beings, etc.) or Asimov's universe (where it's really ever explained how anything actually works), I wouldn't call realistic. Not more so than e.g. Star Trek for example. Ultimately they all take huge liberties as far as the science is concerned. But again, that doesn't make them bad, it is necessary for SciFi after all, to go beyond what is realistic in order to tell an interesting speculative story. It's more important to keep the balance between what you need to get your universe to work, consistency and the amount a suspension of disbelief that you require from your audience just right. As I said, I think ME started out with a very good balance but deteriorated a lot after ME1, IMO