I don't know why others don't like them but I personally dislike them and Express because of false advertising.
They make it seem like a vpn protects you from everything online, from hackers, phishing attacks, viruses etc and provides absolute privacy no matter what. This is not what a vpn does.
I also wonder how they can get away with marketing themselves as a way to get around geoblocking. I don't personally mind this part but I assume it's a legal gray area for them so they do it until they can get away with it.