Anything to worry about if you use mixers and not doing anything illegal?
I don't think it should be a problem to use a mixer while you are doing nothing bad, but the best practice is to do your own research about the mixer you are going to use. Because, sometimes the mixer you might be using is involving in some kind of illegal case or some words is out from the officials on news or media, Once you read such types of news then the best practice is to avoid using those mixers.
Will any one come knocking on your door if you use mixers and not doing anything illegal?
Yeh, it is possible.
Will posts like this be banned too after Jan?
For this you should read the full post mentioned above in the news section and remains there for long enough, the short story is, old posts will do no harm and the posts made after 1 January may harm your account. And you can talk about Mixers but can not ask people to go look for this mixer or search for this mixer etc.
Will using mixers be an expensive?
Yes, they have different rates and different features you can find many on this forum easily.