First, don't compare them to Federer, or go by their stats against him. Nearly everyone is a disappointment in that regard.
nishikori and raonic are already very highly ranked, so i can't call them disappointments.
I think Nishikori has the most complete game (including mentally) from all of the guys mentioned, but he has had injury issues. We will see him take out top 10 players in 2013 in tournaments, maybe even slams, if he stays healthy. He really doesn't have any weaknesses apart from his injuries. He is very underrated imo, probably because he is japanese and isn't particularly tall.
Dimitrov is the most disappointing considering the hype and expectations; too hollywood. Tomic can be loathsome, but has at least fulfilled some of the expectations on his home turf. He's still far behind his hype though and won't pose much of a challenge to the top ten in matches unless he develops his game, especially mentally. The top 10 are starting to figure him out, he needs to get better quick.
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM