I think it depends on the fights. Karazhan seems very doable by a 5-man team, with very few tricky areas. Fights like Kael'thas would be hard to impossible, though, due to the degree of individual movement required.

I think the best example of a multibox-impossible fight is Thaddius (pre-TBC). To beat that fight, you have a maximum of 3-5 seconds to react to random members of your 5-box group getting a debuff, and then move each member individually, depending on their debuff, about 10 yards.

I was trying to come up with multibox strats for that fight, for fun, but the only thing I could come up with would be to create a custom UI mod that triggers the 'move forward' command, and is bound to 30 separate keys (one for each combination of debuffs, not counting 11111 and 00000). These have to be separate due to Blizzard's code model that prevents anything that triggers movement from being changed in combat.

You could then make another UI mod that displayed a grid of 30 boxes on your main, and would automatically highlight a number depending on which combination of debuffs actually happened. You could then press that key to cause the right characters to either move forward or stand still, assuming they were all lined up perfectly.

But even that depends on your guild using a left/right strat, which not all do, since a north/south/east/west strat works better.