These guys are on top their game

I've had lots of stuff printed in all the latest and greatest materials.

If they're strong enough, they're porous and would leak. If it deals with heat, it's brittle, if it's chemical resistant it lacks strength.

Printing manifolds is a waste of time IMO. But, it's possible to print (or machine from sign foam, or other suitable materials) a core which can be used in various sand casting processes.

V-process casting is a low dollar way to sand cast custom items. Leave some stock for machine allowances and treat it like any other manifold. The process also supports cores for the runners and plenum. Takes some careful Design to limit the amount and number of undercuts, but nothing too limiting.

It's a project I've contemplated more than once. Even got access to a 3d scanner that I might try and run over an engine or intake at some point..