I'm now on my 6th crusade & everyone has gone belly up so far, desertions.

The best thing I can suggest is a big fleet, overland is simply too slow, I'm not certain of this but if you do not go the shortest route every turn the desertions kick in, & by land one set of troops blocking your path could activate it.

You have time to get a fleet together before joining the crusade, & as long as its big enough you'll be fast getting to the location, & nothing short of an act of god wiping out your boats will stop you getting to the target.

Going by boat was the first time I had no desertions, (until I got beaten at sea & went into high speed reverse, as soon as that happened I started losing troops), so I would recommend, large fleet, then join crusade only when you're about to board.