"The Death of Superman" was a great film and adaptation of the source material (yes, they changed a few things, but not so much as to drastically alter it beyond recognition). "Reign of the Supermen"...not so much.
Part of the problem may be that while the Death of Superman comic storyline was relatively short and concise, the Reign of the Supermen comic storyline was epic in scale with far-reaching consequences for the DC Universe at large. That's pretty hard to cram into an hour and a half, and even with a pretty cool final battle scene, they're not successful at conveying any kind of major or lasting impact.
The villains don't feel right either. One comes off as rather whiny, another is just completely ineffective. And characterization in general are rather shallow. We don't get to know these Supermen well enough to really care about what happens to them.
While still better than "Superman/Doomsday", "Reign of the Supermen" is still a pretty disappointing follow up to one of the better DC animated movies.