Anything to worry about if you use mixers and not doing anything illegal?
Yes, it is a custodial service, you lose custody of your coins during the mixing process, that is something to be worried about when sending your coins to a mixer. Use CoinJoin implementations if you want a privacy solution that is self custodial.
Will using mixers be an expensive?
I don't think there's any reason why they should become more expensive, they would charge the same service fee as they are doing now.