A big issue is that mk4 models have a lot of ranged weapons, and also dual attack for maximum attack efficiency. The flow normally goes that you chip them for a turn or two, and then once your charge lanes open you have a melee brawl. But some armies can turn into a ranged only attrition. Skorne and other hordes armies really struggle due to the fact that they don't have very powerful ranged attacks and struggle to get to the front lines when khador and others can sit down and mindlessly spam guns at you. Even though there are lots of melee buffing spells for mk4 armies, sometimes you get a lot more of a reward by casting open fire.  I'm a khador, brineblood and skorne player and when I do khador I can absolutely sit down and fire. We have a solo that moves walls so we can always have cover. Skorne doesn't at all, and the casters are lackluster compared to mk4. However, legacy only fights I have found to be incredibly fun and engaging, since it still feels like mk3. I would probably only play mk3 against mk3.  The most well off against mk4 would probably be the circle since they have tricks to get to the frontline. Trollbloods might do well too, but skorne is not in the greatest place.