One problem with only a roof top is people at street level can forget you are there. Especially if the food and atmosphere don't create a unique buzz to promote the place via word-of-mouth/social media.
At River and Post and Cowford, the rooftop benefits from having the main attraction below that can send patrons upward. Unless you are the River Club (not a rooftop but above all other roofs

) atop a building with a one-of-a-kind view and a 1,000 or so workers in the floors below you (at least before COVID), being on top of a mostly office-only building seems difficult to overcome.
I was there once right after they opened as an invitee of others but forgot about them being there after that.