Speaking to the progression question, I prefer the metroidvania approach the best.
The oddities from underrail isn't bad, but it still doesn't make enough sense, e.g. you find some nicknack and it gives you exp? What?
What if instead, you find a serum, and that serum gives you an attribute point to spend how you like.
Or how about you find a really rare book with some "ancient secrets" that gives you a few points in skills for you to distribute?
I'll officially coin it as the "metroidvania progression system", you can quote me on that!
With other systems like how bloodlines does it, rewarding based on quests, I don't really like that either, it has some of the problems that the traditional system has, though not as bad. It essentially creates a problem in that your character becomes stronger, whilst the world remains "flat" type of issue, and trying to "upgrade" everyone else is a nightmare in terms of balancing and narrative etc...
You might be thinking though "yeah, but then why isn't everyone else getting stronger by finding these serums or books either?" It's because it doesn't hit as hard. Finding these items you can easily hide it within the story, such as the player character is just "lucky" so they end up finding a lot of these items, or just tie it into a specific reason, like the player character has a special ability that allows him to "extract" or "use" these serums, that's why he's growing stronger so quickly compared to everyone else.
From quests though it's iffy because quests are a lot more common, they're a lot harder to mask it, and therefore feel more artificial in that regard.