The biggest problem is getting the tip on the top of the valve just so and correct. This give fits to people. Sometimes, some things are easy for some and sometimes it is a royal pain for others.
Only way to get this correct on a rocker shaft motor is by rocker shaft spacers or not to set the valves too deep in the head. Unlike stud mounted rockers, pushrod length has no effect on this. Like rumble said, it can be a pain.