[Jim E]

I don't want to get into symantics here, but a 20A relay will burn if it is pushed beyond its limits. One stuck ball and the FP motor will push the current limits before it too burns out.With a relay, there is no stopping the destructive current and possibly both devices will be lost. A speed controller handles the current surge better and may go into protective shutdown rather than release the magic smoke.

You mentioned wire gauge, the fact that FP motors spec a certain guage makes it mandatory to use a device that can handle the current required of that guage. Sure, it may only draw 15A, but catastrophic motor failure in a stall condition may draw upwards of 50A from the FP motor. Avoiding wiring fires is always desirable. Better to plan ahead of the safety inspectors than to hope you won't be caught in violation.