Are there other effect types which needs special handle here? Could it be some generic rule here about this?

Some comment in the code about such a case wouldn't hurt at all. And maybe make it an utility function or maybe even add a flag to effect typeinfo about this.

Afaik, those are the only two types (others either are self-contained, like color or text generators, or have specific dependencies through seq1/2/3 members). Will add another _tx_test func for this, indeed better than keeping this check just here.