right, but even if your creators a bad actor, keeps everything and seeks out the mints theyve done
they couldn't pull the rug on everything. just whatever they've physically intercepted.
for example your vanity coin from willi, you trusted him with this process?
I reached out to willi to offer bulk splitkey generation for this project and he denied.
now add in a factor thats more in your hands (split key generation) and you have more doubt?
lets assume he did splitkey on the vanity coin.
he might could take your funds if you sent your coin to him with your partial on a coa style ticket.
question a why would willi want it back? it was custom made to you when you ordered it.
personally, I think your coin would be fine with willi,   
as it stands right now. willi could wipe you out with or without the partials.
this process requires more trust in yourself than others.
it does add a layer of security but isn't foolproof. It's a hell of a lot better than what I see going on now.
I'm sure any creator that knows how to make keys wouldn't struggle with this concept.

vanitypool used this concept for vanity wallet bounties. its tried and true.