Rolling with what Tristan said about Milan being quite easy to defeat, I've found this only to generally be the case when you start out very close to them, typically sharing borders. France and HRE are among the factions positioned to reach Milan very early in the game, and as such when you play those factions Milan seems largely like any other faction you encounter early in the game in any campaign. Milan really only becomes a powerhouse when you're playing a faction that reaches Milan in the mid or late game stages, as its had a lot of time to beat down its AI neighbors by that point and rake in obscene amounts of money from its starting provinces, at which point it is often fielding a lot of stacks. So in general I would say that the superiority of Milan is only really a big factor when it survives into the mid to late game stages. You might get a good feel for this if you play Portugal/Spain and set about conquering your way through Europe. Russia may be another good example if you're trying to ensure you encounter Milan once it's had adequate time to really develop. Even a faction seemingly as close as England often gives Milan a chance to gain momentum, as England is often more concerned with taking over the isles before it starts pushing through Europe.