Alexti,
I agree with you, things change when global battlefields spells come into play - in my experience it's rather after turn 40 (but I'm in peaceful Old Europe, so maybe ..
).
But when this happens Caelum gets no special drawback - their mages won't survive RoS (does Mistform not protect ?), but neither will most other mages and troops : if Caelians have SG/IC/TC they won't be scratched.
Same goes for most global BF spells, once they're in summons are needed.