the WAV drive motors were designed to have a "high egress rating", meaning that it is pretty difficult for anything (like a paper clip) to get into the motors, but that means the motors are pretty well sealed up and do not have much ability for the dust from the brushes to get out, and the brushes wear out, the carbon builds up, and the brushes are not what I would define as "overly robust" especially in the brush springs, so brush spring breakage is a pretty fair concern too. I would also say that the Curtis controller fitted in these is a very sensitive controller to shorts or other types of stray electrical signal, which reads a build up of brush dust as a potential short.
Also WAVs were designed for a very flat and smooth floor (retail big box store, as a replacement for ladders on the retail floor, not to replace order pickers or golf carts, which seems to be how a lot of people wish they could be used), so in a warehouse with a few floor drains and cracks in the concrete, a wheel getting caught and then the operator trying to drive out and stalling the drive motor until it opens/burns a bar on the commutator or expands the armature, happens more often than it was expected in the original design specifications.
I would also say that poor battery maintenance causes a lot of earlier than expected drive motor failure in these units.
these are just my experiences in working on these
I think a generation 2 WAV with A/C traction motors and controllers would cure this part of the concern with these. but for now we get another drive motor.
I am always surprised that anyone fixes their WAV with a 3rd set of drive motors, and they always wish there was "another alternative". I see a few companies answering this 'demand for a product ' with 'low level order pickers' and smaller 1 man motive powered aerial lifts.