What I noticed about Kucoin is that their withdraw fee was too high for me. Far higher that other exchanges that I am using.

Another thing is that they started to implement taxes. This one is the main reason I stopped using the exchange.

About the bad review, check other exchanges, you will noticed that they are all reviewed not that good on Trustpilot. I do not know the reason bit the exchange was not bad when I was using it except those two things I mentioned.