Well, i don't think Felidar Sovereign is unfair in powerlevel but I do think it is extremely boring. It is either a Card that does nothing or a card that just ends the game. So the real question to ask yourself is not "is this unfairß" but rather "is this fun?". If you think it is, go ahead and try it out. But if you have a regular playgroup and people there might not like having upkeep triggers abruptly ending games, maybe thats respectable too.
UR Mizzix of the Izmagnus ~~~ Build your own win-condition: Finite Spellslinging
UR Brudiclad, Telchor Engineer ~~~ We are the Borg. We will add your biological and technological distinctiveness to our own.
WUB Oloro, Ageless Ascetic ~~~ A Guide to dying slowly
UBR Marchesa, the Black Rose ~~~ Marchesa's undying Marionettes
RGW Mayael the Anima ~~~ All Hail the Big Chungus
GWU Chulane, Teller of Tales ~~~ Permanents Only ETB Shenanigans
BGU Sidisi, Brood Tyrant ~~~ Sidisi's Restless Servants
WUBRG The Ur-Dragon ~~~ Dragons eat your face