The biggest issue is consistency, many will try a flinter and not do great right away, this will usually result in them reverting back to a cap lock because it is easier to shoot reasonably well.
If you really want to do well shooting a flint gun then you must force yourself to shoot the flint exclusively, even if your having a hard time of it at the start, it will come to you and get easier to hit well with the flint.
My first range session with a flint rifle was great while shooting off the bench, when I tried it offhand it was laughable, really. I spent a couple years placing poorly in the monthly shoots with a flint rifle, then I gradually improved but I never strayed from the flint.
Today I am pleased with my abilities and have no problems going head to head with the cap gun shooters.
There is a certain mystique about shooting a flint gun well, there are many that cannot figure it out and more still that are downright befuddled by the mechanics of it, a flint smoothie sans rear sight is a whole 'nuther demension.
If you stay the course you will prevail.
:v