I gathered a few thoughts after thinking about it for a bit.
There's no doubt that it can absolutely break doubles and squads with customs on. With Fox's Amplyfying Reflector's multiplying reflected projectiles by 2.1, and with the recalled Pikmin having 2 separate hitboxes (meaning they can hit the reflector twice), Olimar can just run around with essentially OHKO Fairs, Fsmashes, throws, and purple side Bs to instantly threaten the other team. The Pikmin's frailty is completely solved by Hardy Pikmin Pluck, so the only way to stop this strategy is to prevent it from actually happening. In doubles, the other team has a chance given how light Fox and Olimar are, but it's potentially nearly impossible in 3v3s and 4v4s because they have an extra teammate(s) to back them up.
In singles, I think it's more limited in its usefulness. Given that the Pikmin can be killed in a strong hit and that damage incurred isn't too significant barring whites, the opponent can just go for a fast and powerful attack to knock them out rather than try risking reflecting Pikmin Throw. However, a situation where this could be put into play is if the opponent is in close range of a Fsmash while a Pikmin is latched on or is running on the field back to Olimar. The opponent's option to reflect a Fsmash is risky because if Olimar predicts right at whistle at the right time, then the opponent would have to deal with suped-up attacks until he gets KO'd or the Pikmin die themselves. Of course, the opponent has every other choice that every character has in close combat; Fox, Mario, and similar characters put up close range pressure for Olimar anyway (who can get punished by Order Tackle's long endlag), and Palutena would use Super Speed in a customs environment rather than the barrier. It does punish those who get caught unaware of the option for even a moment.