Across formats, all versions of Niv-Mizzet, Reborn decks either naturally meet Jegantha's restriction or can with very slight changes (mostly to their removal package, as the Modern and Pioneer builds often play a copy or two of Supreme Verdict)
In the physical card game, having extra commons is handy because players can build more than one deck at a time without having to switch cards out
that is, it makes tokens for dealing combat damage to the opponent
Each one is a colourful doodle of shapes, which fits really nicely with the full-art nature of the cards themselves
Counterfeit risk is LOW for individual Generations singles card values don't justify sophisticated counterfeiting
The benefit to Lazav is he doesn't remove cards from your graveyard, but the downside is you still have to pay mana to turn him into a monster, making him less of an escalating late-game threat