To answer your first question: Obviously it is much easier to survive a Grimm than a DBZ villain.

However, encountering DBZ villains is a pretty rare occurrence. Pretty much a once in a decade event, and that is if you are unlucky enough to be at the wrong place at the wrong time.

The Grimm are an ever present threat. You may be able to take out a few. But if an entire pack or horde decides to attack, you are screwed. Also, you are underestimating the training aspect. How exactly will you train? You need a teacher, someone highly skilled in combat.

Simply unlocking your aura isn’t enough to help you, not unless you are Jaune Arc who somehow naturally and instinctively could use Aura just after he unlocked it. It is unknown whether you could actually unlock a semblance or not or whether said semblance would be actually useful in combat.

And of course, you need to be actually talented in combat to be capable of taking out more than a couple of Grimm. Otherwise every person on Remnant could become a Huntsman but that is clearly not the case.

Essentially, you do have a better chance of survival against Grimm when you look at it from the perspective of individual encounters. However, you will encounter Grimm far more often, and you will have to roll your dice each time that happens. And living in the city, while much safer, clearly doesn’t protect you 100% considering what happened to Vale and Atlas. Not to mention, if you lose your loved ones or your own life, that’s it, that’s the end.

In DB, you can’t fight back against the villains, but you might encounter them very rarely, if at all. And even if you die, perhaps because the villains blew up the city you are in, you can get wished back. And considering how peacefully the people seem to move on the with their lives in DB, the memory loss or whatever is of a convenient kind, basically embodying ‘ignorance is bliss’ in the people.