They released their fantasy RPG first, and later games were not exactly compatible; but in the years afterwards, they did start to revise them for compatibility. These days they are calling it Megaverse
tm, and they are for the most part now all compatible. Originally some characters from various games did not stack up well with others; sometimes because of rule differences.
[Rifts has even HtH attacks that do HUNDREDS in damage, where Fantasy RPG HPs usually number in the tens at most.]
They also released a supplemental book with specific conversion rules for their various settings; including the Fantasy RPG, and Ninja's & Superspies, and TMNT.
As for 'Heroes Unlimited' though, I thought it did a really good job on character creation and power categorization. Balance is debated by some. You can make an X-man Colossus and an Avenger's Hawkeye... and easily argue that Colossus greatly overpowers Hawkeye.
I suggested it as a resource/guide for working on a superhero RPG [as opposed to an action-Sim like 'City of Heroes'].
There are weapons charts for guns and melee, and the damage dealing superpowers are all worked out to be comparable.
It has the concept of major and minor super abilities; these being limits to prevent everyone from making Superman.
Generally you get more powers with multiple minors, than with one major, but the majors are significant; Sonic Speed can directly apply to melee combat!
