well they do got one thing going for them that a lot of others don't you can still easily buy new parts for the majority of their lineup cheap.
Like if you broke a part on your 30 year old stampede(yea scary the stampede is that old) can get a new part pretty much any where RC parts are sold for cheap. Me broke a part on a 20 year old HPI model I had to pay 3X MSRP to some guy on ebay for one of the last 5 for sale on the planet >.>
I paid the same for a set of bumpers(new front and rear) for my 12 year old slash 4X4 then I did for the diff cups for a MT2

While their quality is kinda meh their parts support is top of the line
Just my opinion though.

as for the maxx though... no idea
My guess is those tires have too much rotational mass for the diff, and when they land with throttle they rip it up. The tires look real big, and heavy. This is just a guess based on looks though
EDIT: Just looked at a video and seems it is more of a drive line problem that would be fixed with a slipper or a center diff. Until someone(or traxxas) makes one of those I wouldn't touch it.