On a small map, before the mages can develop, they'll be wiped out. And if you save up an army and waste time, the Necromancer army will grow many times over.

Well, purely theoretically on a map of size S, the Tower has excellent chances of victory — shooters and fast gargoyles are an option for rushing. Purely theoretically, the Conflux has the same chance — little fairies with no retaliation and level 2 shooters. BUT — there's a nuance — for the Conflux, this will be MORE EXPENSIVE — that could prove to be the deciding factor. Necros, however, on S have, IMHO, the lowest chance of victory (out of these 3)
But on M — the Tower takes a long time to develop. Here, the chance of losing to both Necros and especially the Conflux is quite high!
L and XL are already better! There's time for development. And a developed Tower is terrifying!