You can only use amiibos against friend, when you're with friends you have a sort of code between each other of what you can and can't do, also there are toggles for it so you can turn it off.
Also you kind of posted this thread assuming that we knew you're talking about smash bros.
By no means are amiibos pay2win DLC.
I would say that this is their answer to teaching new players how to get good, without just having a system of: play against level 1, beat them, go to level 2, etc.
These are CPUs that learn to play against you, so imagine that you have a blank enemy, how do you beat him? You hit him off the edge ofcourse.
But now he has learned to knock you off the edge, so you need a way of being better than him now, so you need to learn to charge your smash attacks.
Now the CPU knows about doing that, so you need a way of improving over that, teaching you to explore all the mechanics at a steady rate without guides.
And then when you're amiibo is a very powerful CPU, you want to see what happens when it faces off against other people and how they react different and change up the playstyle of the CPU, which brings you into a whole new level of strategy which is how others play.
Most importantly regarding your concerns though, I would say that having an amiibo on your side is almost exactly like turning on a CPU and putting it on your team, which has never been a problem in the metagame of smash.
Oh and amiibos being allowed in 'For Fun' Mode? Noting of the sorts have been said about that, and it wouldn't make any sense for that to be the case, since 'For Fun' Mode is 4 players, which is the maximum amount that you can have on screen at any time.