You have to sacrifice a creature every turn to keep it, which, in a token deck, is a very small price to pay
They are nearly impossible to contain and highly intelligent, featuring physiological and neural capacities similar to humans
Gaining a level won't remove abilities that a Class had at a previous level
Then make some mana with firebending four too
Once you start playing two or more of these a turn you'll start seeing absurd turnouts, and they're actively busted with mass reanimation like Raise the Past, Return to the Ranks, and so on
It just sits on the battlefield growing absolutely everything, and if your opponent can't deal with it in a turn or two, they'll have already lost