the backup guardians is basically the point where i would consider bothering with possessing fixed bosses since most of them suck
like you dont want to ignore possession entirely since you can still get weapon mastery from forms if nothing else, but forms are really weak on normal/NM since enemies in general are weaker
Well, you still can get some really nice forms from vaults in the easier settings. In my level 97 Champion of Urh'Rok form - I had 140 physical power, all saves above 75 (physical almost 100) and 5200 HP. I didn't lose a single hit point fighting against Aeryn and the two sorcerers and less than 200 HP against Atamathon. I also had a level 108 Forge Giant, three Multi-Hued Wyrms over level 90 and a few other nice forms left in the storage that I never had to use because I didn't encounter any enemy that could put a dent in my Champion of Urh'Rok form.
The Boss form I got most use out of before that was Warmaster Gnarg - good stats + Windblade and Giant Leap, that was more than enough to easily clear any vault in the Prides. The backup guardians were underwhelming except for the Orc in Daikara - around the level you fight them, you can also easily find high level Fire, Ice or Venom Wyrms which are all much better than any of the backup guardians (but I'd always take Snaproot, not to assume his form but rather for healing a better Boss form).