Demo accounts or let's say virtual tradings are kinda meh for me. It's not really where you can build skills or make better decisions from your trade since it's not your money and you don't care nonetheless on the outcome of your trades and this is why I didn't use those back in the days. If it is your money, you have that psychological mindset of being careful on what you do with your trades unless those money aren't really practically going to be needed anyway.

Reliable exchanges obviously won't give your profits, but it will keep you away "somehow" from being locked out from your account. Can't judge what exchanges are reliable these days, since I had quit trading long time ago and what I am pointing out in here are just purely from experience.