I'd say DA and Sisters have a hard time.
First off every army is tailored to screw over 3+ save armies that are far more powerful (Trait Marines, Old Chaos), so do even better against these.
Second your tactics must be different depending on opponent.
For Tau, the tactics are the same regardless of opponent- you try to stay out of combat (very easy with JSJ and mobility) and shoot the high priority targets first, working down the list. Very straightforward. Once you've worked out a competitive list (clue, min FW's), your playstyle is the same every game.
For Eldar, it's spoonfed- this is a unit of Fire Dragons, it kills tanks, send it at tanks. This is a unit of Harliequins, it kills Marines, send it at Marines, etc. Each unit is specialised to do certain things, so just position them to do those things. Never understood the Eldar player mentality that it is some superior act of skill that creates synergy- no it's having the hardest tank in the game and units that are Paint by Numbers.
Whereas a DA Tactical Squad needs to do everything, do you move forward to get into combat, do you sit right back and fire the heavy weapon, do you move into a position where you can fire all weapons? All depends on opponent, enemy units positions, etc. And though they can do each satisfactorily, they can't do as well as a specialist unit in it's given field.
Of course Grey Knights and IG are far harder. IG for example are a static, weedy shooty army in a 4th Ed ruleset dominated by Near-Fearless MEq's, Highly Mobile objective grabbing armies, etc.