“It’s just not that good”

That might be true, but it’s worth being clear about it.

Not that good for who?

If you mean to say, “I don’t like it, it doesn’t appeal to me,” then that’s what you should say.

If, on the other hand, you have enough expertise and domain knowledge to say, “I understand what has appealed to the audience you’re trying to serve, and this isn’t going to work.”

The first is easy, and perhaps not that helpful.

The second is priceless, but are you sure?