In my Venice campaign I used one third stack with general, few mounted crosbows and few knight hopitallers (which btw. nullify italian supposed lack of good cavalry). They managed to conquer few cities (general hired few mercs to push siege equipement) and won lot of small field battles (IIRC never encountered full enemy stack) before they suffered loses that made them go beck to replenish their numbers.
From this I modeled armies I used in north africa (lot of land to cover).

So my perception of mounted xbows is that they are REALY strong (but probubly any normal ranged cav would work in this example).

btw. are HRE's pistol using cavalry worth using? I guess that they could be better than m-x only aginst oponents with shields (I somewhere read that both guns and crosbows are armor piercing, but guns skips shields).