There are two visions of fun, objective fun and subjective fun, the objective fun says that for something to be fun, it needs to do something in particular (positive feedback, skinner box rewards, flow state between challenge and skill and etc) and subjective fun where fun is completely dependent of the perspective of the person, if you see something you think it is of value, automatically you are engaged and find it fun (I can see a nihilist, depressive film as The Joker and find it fun). The opposite of fun is boring, boring means the person, at minimum, is not engaged. When someone says that he didn't find something fun, that means, the product failed both the objective AND subjective criteria for that person, that person wasn't engaged at all.
So, when someone say something was great but not fun, it is very odd, you need to question that, why that person wasn't engaged? was the gameplay concept interesting and with potential but couldn't hold up to 90hs? Was the story not enough to motivate and give context to the gameplay? Was the story weak? If a review don't explain that, it is useless. A game could have very interesting concepts but fail to properly explore those concepts. I could see someone seeing those concepts as great but not fun. However, giving 9 or 10 to a such flawed experience seems like "I want my hipster credit." to me.