In effects that use the Gilbreath principle, for example, you can let the spectator riffle shuffle the cards. But they can shuffle only once. How do you prevent them from doing a second riffle shuffle without arousing suspicion that you didn't let them do another shuffle?
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Its all about attitude. just be casual and act like you don't care about the cards. ask them to shuffle, after they have done tell them to also cut the deck. then reaach out for the deck and they will happily give it to you.

I would never take that risk.
You also have another problem - how can you ensure they do a riffle shuffle rather than overhand shuffle (which is much more commonplace, at least here in the UK)?
I would not want to ask a spectator to shuffle in a particular way - it is too obvious that they can't genuinely mix the cards.

So - for me - the solution would be to do the riffle shuffle myself. Adding the convincer of spreading the deck to show them genuinely interlaced, and asking the spectator to push them together.

OR I would use a completely different "random" way to mix the cards - that would be equivalent to a riffle shuffle but look totally different. EG. cut the deck into two halves and have two spectators deal bunches of cards into a single pile to "prove" they are mixed. This puts you in control much more than just handing the deck over to be shuffled.

Lay the cards out for a Rosetta shuffle