I play Hakan, Hawk, Dan, and Dhalsim, and of those 4, Dhalsim is the worst imo.
Major problems:
--Weak antiair: Sim's antiairs all require startup time and all take a big commitment. I can't throw a fireball and then use my invincible dragon punch if the opponent jumps; if I don't have enough frames to start a back+hk, back+mp, ex blast, etc, I have almost nothing. Lots of characters have jump attacks that beat Sim's normal antiairs if they get the timing or spacing right. If they get the jump in, they take 1/4-1/3 of my life and I'm put in a bad situation, while if I get the antiair, I deal like 90 damage, so the incentives are not in my favor. I can do a u1 for the opponent to land on, but if they empty jump, I might be screwed, and my comeback potential is screwed without u1 as well. And if an opponent character can change its jump arc, then Sim is totally screwed because all his antiairs require big commitments. If I see Seth jumping at me and I do back+mp, but he delayed himself slightly with divekick startup, then I'm totally screwed. Or if I see Cammy jumping at me and I do ex blast, but she shortens her jump with divekick, then I'm totally screwed. Too many characters can do this.
--Weak damage: Nobody is scared of getting punishing by Dhalsim. People do otherwise stupid things like random dps and random jump ins and random focuses because they're just not worried about Sim's potential damage output.
--Weak mobility: Even with a teleport and instant air teleport, Sim's mobility is weak compared to much of the cast. His walk speeds and dash speeds are awful, his slides are all very negative on block, and his drills and mummy are slow and don't cause much hitstun or blockstun.
--Bad up close defense: There's a really frustrating range not far in front of Sim where he doesn't really have the tools to deal with the opponent's offense. Nothing invincible, nothing damage, no combos without counterhit, no fast or invincible focus breakers, etc. This range sucks.
--Snowball effect: With my bad antiair, I'm susceptible to getting jumped in on. When the opponent jumps in, I have bad mobility and bad up close defense, so I'm screwed.
--Risky offense: Sim has such trouble zoning in some matchups that playing offense might be preferable. He has good safe jumps and crossups and frame traps actually, but the risk reward is not in his favor. Oh, I mistimed a safe jump? I eat dp fadc ultra and get knocked down and now I'm screwed. I guessed wrong in frame traps? I get knocked down and now I'm screwed. I guessed right? I deal minor damage. Also, making comebacks can be really tough. He only has a couple tools to force mixups, like u1 and in some cases ex fire. Without those, the opponent basically has to let himself get hit.
I don't want all of those weaknesses fixed because then he'd be too good. Sim is getting more damage in USF4, so hopefully he can make people think twice about some things and play a bit more effectively on offense. But his antiair, up close defense, and mobility still suck.
I feel like he'll be a little bit better in absolute terms, but relative to most of the rest of the cast I think he still sounds weak.