There are certainly devices that are harder. Many relies on various bugs. Most ICS and Jelly Bean devices appears to be trivial to root due to a timing issue in the "recovery" functionality, though - there's a script floating around that claims to root pretty much everything.

Note that there are several issues: Rooting is often reasonably easy. But rooting is not enough to be able to install custom roms. For that you also need to be able to unlock the bootloader. There's any number of obstacles.

That said, there are tons of easily rootable and unlockable devices out there, so it's not really a big problem for those of us who insists on one or the other.

Especially with more and more higher end devices coming out of non-descript Chinese manufacturers who couldn't give a crap about the carriers desires, and which will start being "good enough" for more and more people (my new phone is Star phone that looks exactly like the Samsung Galaxy SIII, and while it don't perform as an SIII, it beats the pants of my 2 year old HTC Desire HD in every way and cost a fraction of what my Desire HD did)