You will generally find it harder to work with "low-end" mechboxes in terms of re-assembling them, they like to put up a fight.
I worked on WELL Version 2 Gearboxes, like the first generation ones and man after a night of working on those my fingers would kill. But when you work on a Systema, G&P, or TM you'll be done in almost no time. Jing Gong mechboxes haven't given me much trouble, only if the spring is really strong than some measure must be taken to ensure it doesn't go flying but that all comes with experience.
If you need to ask if the anti-reversal latch is important because they can't get it in should really re-consider charging people for services related to FIXING guns.