It was created
specifically under Thatcher to serve otherwise under-served audiences, and that inevitably means it's controversial, because if what it showed was
not controversial it'd be on one of the channels all those well-served audiences are watching.
It's most reasonably criticised for making too "easy" controversial choices, not really challenging for an audience, not really telling anybody anything they didn't know, just going for outrage - something like let's show people having sexual intercourse on TV but with medics explaining what's going on, rather than satire like Brass Eye that demands a bit more of the audience.
So for example Queer As Folk, now considered a pretty tame TV series (and indeed since rebooted for a wider audience) about young gay characters in Manchester was sufficiently controversial when Channel 4 first aired it that right wing newspapers of course insisted it should be banned. Arguably this was "too safe" but on the other hand it's not as though other broadcasters were doing this at the time.