Ah crap, I just looked it up and, while XYZ Dragon Cannon was an obvious no go, Valkerion & RE Metal Dragon can't be summoned by this card because they all have Summoning Conditions.
-- XYZ needs its Tag/Combine Fusion thing...also can't be SS from the Grave (Banished is OK)
-- Valkerion & RE Metal needs to be summoned normally at least once.
Graveyard Fusions seem to be OK, but like 0 mentions of Rituals are cool too.
....are there any Ritual Machine/Rock Types? O.o
The only monsters currently obtainable in Duel Links that you can use Magnet Reverse on are Valkyrion, Red-Eyes Black Metal Dragon, Machina Cannon, VW, VWXYZ, UFOroid Fighter, Panzer Dragon, Labyrinth Tank, Master Diamond and maybe Gradius' Option (not sure how that works in this scenario, but it lacks a hard Summoning condition, so would otherwise be possible).