You're going to spend at least 2 monsters to summon Doom Dragon, most likely 3 due to how low the Level of Infernity Tuners are, unless we get Launcher (which is very unlikely) you most likely won't have any good monster left to attack with.
"most likely won't have any good monster left to attack with."
"This card cannot attack during the same turn you activate its effect"
" You can only activate its effect if you have no cards in your hand."
Some genius will show us a way to work around those disadvantages. Besides, at this moment, we don't know whether Infernity will get useful support cards like Launcher (or even better cards).