I guess that's just a part of the competition. That's the reason why gambling sites have to develop their sites continuously. There will always be negative reviews but only the site alone could prove if it's not true or if they're really honest and giving good services to their users.
It is rumoured that most of these review sites are owned by online casinos and they pay people to give good reviews of their sites. You have to remember that these sites are not regulated like most of the brick n mortar casinos, so it is bit of a wild west out there now. <everthing goes>
Bitcoin's annoymity are helping these sites to hide most of these activities, so you should not even bother to take reviews on these sites seriously. I would rather suggest that you signup for these casinos and test them for yourself.