"Not a good cultural fit" always strikes me as similar to "gut feel". It's an itch that something isn't quite right without saying (or often knowing) what it is.
To me if you think that someone isn't a good cultural fit you need to dig around and work out what you mean by that. In most cases there's something there and it's not something you can realistically change, but in some cases you'll find something worth working on.
Not all problems can be solved, not all problems should be solved (companies often "stop people from doing the job" for good reasons - a company can't be all things to all people and what is right for person X may not be right for everyone else) but "not a good cultural fit" is a bad because it's not a specific problem anyone can address. At the very least if you're going to get rid of someone (or not employ someone) it seems fair to let them know why.
As an aside in the UK "not a good cultural fit" is also potentially going to get you into legal trouble because of the possibility that it's hiding racial or other discrimination (for instance if "not a good cultural fit" means "we all like to go drinking, you don't drink because of your faith so we're worried you won't fit" that could well be considered discrimination).
To me if you think that someone isn't a good cultural fit you need to dig around and work out what you mean by that. In most cases there's something there and it's not something you can realistically change, but in some cases you'll find something worth working on.
Not all problems can be solved, not all problems should be solved (companies often "stop people from doing the job" for good reasons - a company can't be all things to all people and what is right for person X may not be right for everyone else) but "not a good cultural fit" is a bad because it's not a specific problem anyone can address. At the very least if you're going to get rid of someone (or not employ someone) it seems fair to let them know why.
As an aside in the UK "not a good cultural fit" is also potentially going to get you into legal trouble because of the possibility that it's hiding racial or other discrimination (for instance if "not a good cultural fit" means "we all like to go drinking, you don't drink because of your faith so we're worried you won't fit" that could well be considered discrimination).