Anecdotal evidence on both sides of the argument, but I found no difference in difficulty of adjustment between the Shimano groups I've had experience with. The 10 speed 105 I run now on my main bike is easily the most reliable I had so far. I'd say adjust difficulty was the same for all and I would describe it as easy.
W.R.T Shimano, I've used following quite a bit:
Downtube:
7 spd : Exage, RSX, 105
STI Brifter.:
Ultegra 8spd, Tiagra 9spd, Ultegra 9spd, 105 9 spd, 105 10 spd