I've found it's exactly as easy and troublesome as any other commercial grade simulator.
All of them, from free 3f5 SPICE, to $10k+ Altium's SPICE engine, are all based on either the original Berkeley 3f5, or the somewhat later XSPICE (also an academic byproduct; Georgia Tech I think??).
They almost all have custom tweaks, like PSpice's proprietary notation (syntax, built-in functions, and probably convergence tweaks), Altium's partial support of many things (including some PSpice, and maybe just a hint of HSpice), and probably most significantly, LTSpice's heavily tweaked solver ("modified TRAP", actual real no fooling multiprocessor support, etc.).
In short, it doesn't matter what you're paying (or not), they all suck uniformly, just in different ways...
Anyway, not to continue into the relative differences of the various packages, I'll simply close in saying: no, there's nothing fundamentally different about Altium's. It's harder to drive, but the convergence is just as good/bad as the user sitting in front of it.
Tim