Although amiibo have seen much support throughout the Super Smash Bros. series, it has become clear that the developers never intended a complete metagame to form around them; as a result, there are many aspects of amiibo play that are considered overpowered or broken. In both Super Smash Bros. 4 and Super Smash Bros. Ultimate, there have been many bans on characters and equipment.
In Super Smash Bros. 4
In the Smash 4 amiibo metagame, several characters and equipment items have been banned. Critical-hit capability and Explosive perfect shield are entirely disallowed and cannot be used with any compromise; Little Mac and Cloud, who were both once entirely removed, are now allowed with certain restrictions on their equipment setups.
Little Mac, with no restrictions on equipment, can use a maxed-out Attack stat so that his forward smash can shatter a full shield in one hit. Little Mac often used this “strategy” to end 2-stock games in 20 seconds or less; there were many amiibo who were hard countered by Little Mac and no one character could consistently defeat him. The community at first banned Little Mac from competitive play, but he was later reinstated with the following restrictions: his Attack stat must be 0 or less and he cannot utilize any Attack-boosting bonuses.
Cloud had mostly the same issue as Little Mac but to a lesser degree; with enough Attack, all three hits of his forward smash could shatter a full shield. Cloud was banned for a very short period of time, but was later reinstated with the following restrictions: his Attack stat must be 60 or less and he can only utilize one Attack-boosting bonus.
In Super Smash Bros. Ultimate
Super Smash Bros. Ultimate has not had as many bans and restrictions as Smash 4; the goal is to make the rules as accessible to new players as possible. In the early Ultimate metagame, Olimar was disproportionately strong, as his Pikmin received three times the benefits of Spirit stats. This was entirely unintended (and was later patched out) and often resulted in one-hit KOs from Olimar. The community agreed to ban Olimar until the glitch was fixed (which it later was) save for a small YouTube channel that disagreed but lacked any real credibility.
Currently, the only running ban is on the Super Armor and Slow Super Armor Spirit effects due to their overcentralization in the early Ultimate metagame. In early 2019, many amiibo trainers stopped using Spirits, as they were somehow less balanced than equipment in Smash 4; in particular, Defense points are worth much more than Attack points. This resulted in a heavily defensive metagame that was boring to watch. The community is currently discussing a potential ban or restriction on defensive stats and bonuses, but no conclusion has yet been reached.