My BONE stock 99 with the NV4500 spit the nut at under 100k, fixed with the "updated locking" nut. At 130k or so did it again, also ended up with a hairline crack (not failure) in the gear. Fixed again with the better "updated" nut. Started having issues again 40k or so later. Put in a reman from a very reputable company it broke a gear 50k or so later, they warrantied it. I gave up in another 40k when it started to have an issue.
I always ran the correct fluid, changed fluid, kept truck stock, and simply drove it. Never hot rodded it. It towed 50% of the time loads weighed from 3500-9000 or so pounds.
As for the NV5600 I agree they are getting more scarce and parts are getting more expensive but it is a much stouter trans. You could also build a bullet proof auto for 4-5k too....