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 Megaversetm
, 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!