If you're careful, almost none.
the components are pretty solid, and the unit is modular enogh that its not complicated to take apart and put back together.
Areas to watch out for:
1. The shifter's ball and spring that lock it into the metal plates (lock it in gear) are tricky to put back into place as the resistance mod puts much more force on the spring, making it hard to hold all the pieces together.
2. Don't lose the little metal ball! The spring will shoot it out when you disassemble the metal plates.
3 Pay attention to how the potentiometers on the shifter are placed, you could break them off if you reassemble the metal plate housing wrong.
If you do break them, you can replace them with the potentiometers of a DFP pedal set. (I broke mine as I was experimenting with a mod, and that was my fix)
4. The ralisitc resistance affects the sensitivity of the right potentiometer due to the different level of flexing that the metal plates incurr from the added resistance to the spring, so sequential shifting may stop shifting correctly.
If this sounds daunting, and now you feel like "no thanks", don't worry, if you do it correctly, the shifter becomes perfect!
I've done almost all the mods to my shifter and it feels great. I've had it like that for about a year with heavy use and no problems (after sorting out the issues above
)