Yes, you definitely have to be careful what you send against PCs at that level. It has to be things like a basic skeleton with no DB and a weak OB. But I think the difficulties the players have really teach them the value of positioning, of knocking down enemies, of having debuff/debilitations spells, of parrying, etc.
I think this is especially true of first-time RM players: if they're coming from DnD, they might not appreciate how much more tactical RM is. In DnD 3.5 for example, getting a rear attack was +2, which is equivalent to +10 in Rolemaster. But in Rolemaster, it is actually +35, which is equivalent to +7. It can take a while for DnD players to realize how important it is to get flank and rear attacks, and to deny your opponents the same.