Shimano replaced the 10x1 threaded steel axle they've used for >20 years with a larger diameter, stiffer Al axle and end caps. This was done to appeal to MTBers who tend to jump off things and bend axles. It's basically a marketing appeal move, since it offers no real benefit to the majority of users.

When they made the axle diameter larger, using the same hub shell dimension, it left less room for the bearing assembly. Shimano changed the 1/4" balls to 3/16" balls as a result of this "makeover". Small balls have a lower load limit and roll with more friction. I've read 2 separate reports of bearing-related 770 hub failures. Anecdotal only, but it is consistent with the design change. Shimano should have taken this opportunity to eliminate cup-and-cone ball bearings and replaced them with beefy cartridge bearings. But then their hubs would be completely serviceable with an unlimited life. As they are now, the pressed-in cup eventually gets a track eroded into it from the balls - then you have to buy a new hub and build a new wheel. A cartridge bearing upgrade would cut into lots of profit in selling replacement wheelsets.

As fietsbob suggested, choose Deore or LX (european availablity only) hubs if you want them to still roll as reliably as Shimano MTB hubs of the past 20+ years. Better yet, spend more on some Hope, DT or similar hubs with good replaceable cartridge bearings. Also, note the service procedure for changing bearings before purchase - many require special tools and expensive, odd-sized bearings that make the cost of long-term ownership of those hubs much higher. Hope hubs are disassembled with common tools and bearings are an inexpensive size. Phil Wood is the same in this regard.