The benefits of 5x5 training vary, so the people that would benefit from 5x5 vary. A 5x5 program is a simple routine that is aimed for building functional strength. Thus a great and probably the best benefactor of 5x5 training would be a beginner.
Beginners start off weak and should be focused on increasing their strength first of all, something that 5x5 does best. Add in the fact that the routine is simple and it makes it all the more attractive to a beginner.
The people who would not benefit the most are those that are in a cutting phase. While muscle size and strength would still be gained when doing 5x5, it is not the optimal routine because the low reps focus solely on strength.