A fixed blade head needs to be very straight in order to prevent planing. The faster the bow the worse it can become. Because a crossbow advertised as a 330 FPS bow will pretty much always produce that give or take a little and a vertical bow will almost always produce substantially less because of poundage, draw length and arrow weight you need to pay more attention to it with the crossbow.
Because you have full IBO speed with the crossbow and the arrow weight is generally heavier, there s much less reason to consider smaller cut on contact fixed blade heads. A crossbow with a very large cut mechanical has enough more momentum that it can much more easily drive a 3 inch or larger cut all the way through Bambi. Last year I killed to 1 1/2 year old does with a 3 inch total cut NAP Spitfire Double Cross through center punched lungs at 20 yards, and still had enough snot left to drive it full length into dense sand out of a 330 FPS bow. I wouldn't worry about a 4 inch cut mechanical if I had one. I haven't seen flight issues out of any of the big mechanicals I have tried, but I have seen issues with fixed blades.
Good post - and I'll add that the alignment is not just with the broadhead, the insert and the broadhead have to be as straight in the bolt shaft as well.
A lot of problems are avoided by going to an archery shop with a good reputation for quality work the first time. 99% of my issues with arrows and bolts have happened at a chain store, including Cabelas. Usually it's a young "tech/pro/lodge" person cutting or gluing without proper training. Some stores have stood behind their work, and some have not.
So what?
At the IBO crossbow speeds, ANY variation in the alignment, broadhead tip, or insert alignment will show up pretty fast. If you are not numbering your arrows/bolts, you should be. I was surprised how often the "flier" turned out to be the same one. Usually, when I took my time and looked at the flier, I could find the reason.
Fixed blades have more surface area exposed in flight, and are more prone to "plane - aka fly" as they catch the wind. Mechanicals have less area exposed, but they need to be crossbow-rated or designed, or they can pop a blade open on the shot and then they will plane like a mofo as well.
OP - you are going to have to try some different broadheads with your set-up and see how they perform.
RickyBobby's Montecs are a good compact fixed blade design, but I've had them fly weird with very little variation in the tip and blades in my practice heads. I'd try the Slick Trick standards first, they seem to shoot well in every bow and crossbow I've tried them with.
I personally use the Rage X-bow 100 grain heads, they've not failed me, and have cut stupid large holes in and out. I've changed from the o-ring to the shock collars, no issues yet, but I shoot close range on deer.