Quite honestly depending on the game multiple rolls may be a hinderance or not used at all.
PbP games usually die off pretty quickly as is and the blast splitting means the attacker rolls X number of attacks. The DM then has to keep track of all that to roll individual saves versus each one. Granted not overly hard to do but does add extra stress.

On live games..well not sure how many RL games use homebrew like this (I'd like to think a lot) but it could be only 1 save role as well. I've used that rule often DMing to speed things up. During each round where a save is needed to be made I'd roll once for each type. Its not overly unfair because the law of averages applies so one round it may be immune to them all, the next round it may get hit by every last one.

I do think its a good unique idea though that allows a warlock to be a caster debuffing/affliction machine versus damage.