The biggest issue with the Handler 135 has been that components Q1, PTC1, or CR1 on the control board can fail at anytime. Luckily they are relatively inexpensive components that can be purchased through on-line electronics components suppliers. A redesign of the Handler 135 with the Handler 140 seems to have cured these troublesome failures.
Depending on how it has been treated, the H-10 gun does have some issues with the power cable conductor(s), in the lead, breaking at the crimp connection point at the power pin end.