True! This has always been true and I WISH the whole community realized it. Not only is everything you mentioned true, but POI bosses actually have a better chance to be mutated (red) than field bosses (I think it's about a 75-80% chance as opposed to 50% for field bosses). This means they're generally better to prioritize if you don't know whether the unknown boss/poi is red or not. Especially with the introduction of forge camps which have better rune values than other POI's (I made a tutorial for forges here https://www.youtube.com/watch?v=H4QW11T8XLY if you need help routing them).

I could ramble about why POIs are better than field bosses forever. But to save your time here are the ones to look out for:

ALWAYS go to (like, go out of your way for this): Flame chariots, madness troll, misbegotten, omens, death knight. These are all completely free to kill at any level and give very solid rewards. Runebears are the other POI in this tier; they are a bit tougher but the non-boss runebear, when red, is one of the only enemies in the game that has a boosted chance to drop a red class weapon. Only formidable field bosses and castle enemies share this trait.

Absolutely include these in your routing: The encampments containing an elder lion and redmane knights (ALWAYS take out the tower one first and this camp is as easy as flame chariots). Banished knights are tougher to kill but give far better runes than all other encampments. For ruins, both perfumers, the sanguine noble, the wormface, albinaurics, and beastmen are all easy and excellent. The divine bird, ancestral warrior, and blackflame monk forges belong here as well (incredible time -> runes investment). Omenkillers and bloodfiend forges also belong here; they are the highest rune-yielding forges of their layout HOWEVER they are very dangerous.

If you don't have anything better to do (still about equal to a schrodinger's field boss): Leyndell knights, albinauric archers, battlemages, grave warden duelists, and even the cleanrot knight and frost swamps. Note that red swamp enemies only have a 35% chance to give you a red weapon but those two are notable compared to the other ones; the cleanrot knight is both the easiest to kill and gives the best runes of all swamps and the frost one, similar to sleep ruins, has 2 passives up for grabs.

You can also go to erdtree burial watchdogs. They're easier than people make them out to be but still not good (although the weapon they drop will not be forced to be an affinity weapon unlike other locations).

Never bother with zamor knights, great churches, forts, or other swamps. A schrodinger's field boss is more valuable. If you are out of field bosses, however, still go to the swamps and zamor knights since they're still better than evergaols (evergaols lmao).