RE: Savage decision?
for the MT2 yes they can be fragile(I found the front arms the only issue personally)... Kinda why I stopped using mine(well I bought a 1/8th scale buggy which helped to stop using it), oddly my MT1 was way more stout.
As for the savage flux... Few questions on what your friend is doing to destroy the diffs
1) Is he running it on 4S or 6S? If 6S that is an insane amount of force being put on the diffs (way more than a nitro engine could pull)
2) How is he landing? Is he landing with the throttle on? If so that will toast a diff on any RC if done enough times...
I've not run my flux much (not due to issues just due to lack of places to run it currently), but I have beat the hell out of my Savage X including taking it to a BMX park and jumping it non-stop off 10 foot tall jumps sending it 20 feet in the air(mostly bad landings) for well over an hour straight. I'm only guessing your buddy is doing the noob to jumping thing and landing with some throttle on so the wheels don't get to spin freely. Bad jumps for the most part can cause a lot of damage if you land wrong.
Also its like other RC's I've owned I got a few multiples of the same model which are set up the same, and will find one to be more fragile than the next. Could be something to do with the day the parts were made if the mix was off slightly.