They can work it depends on how they're used Killer Instinct for Xbox One users it well for things that are not pay to win and can be unlocked free with time.

When I used to play CoD I had no issues paying for the extra class slots since I refused to prestige.

I think with free to play mobile games it is kinda hard not to have micro transactions. True, they can make it more fair to players who don't want to pay, but they have to make money somehow right? Games that you pay for and have micro transactions are stupid because you bought the game so you shouldn't have to pay more money but I don't mind a free to play game asking for a few bucks.

It sucks that BO3 also has micro transactions.

I prefer single player games where there is no paid advantage

Almost every multiplayer game has paid advantages now. Even after paying a lot for a game you still get beaten by someone because they spent extra money on the game.