I haven't really had any issues with DHD, since the only thing he can really do in throw his little disk in a straight line and try to use that exploding can and Cowboys. The disk and cowboy you can jump over. And if you Zair the can you can easily turn it against him. And up close we wreck him so yeah...
As for sheik I haven't been able to reliably punish her needles with down b. Just have to pay attention and hop over them or walk and power sheild. This matchup is tough, but I think that played right it's fairly even. Sheik will wreck us at first, since she can fair walk. But she has issues killing, as long as you don't get hit by fish flip gimmicks and dodge those up airs... Once we're at like 90+ percent we can start playing catch up, and generally get first stock, and then get KO'd shortly after, and then it's another game of catch up. Of course you this is worst case scenario. If you can zone sheik out with side b and zair then do it. Because you do not want to go in on her if it can be avoided. This is just my opinion though.