Some 5.3L questions

[Accelo]

The LS motors last longer than any typical 350, especially the ones with a carburetor.
A lot of reasons for this;
Better oils
Low tension rings
Roller camshafts
Better control of the air fuel mixtures
Better ignition controls/wiring
Improvements in metallurgy.

Of these the "better control of the air fuel mixtures" is likely the most important.
No more washing cylinders with fuel and dilution of the oil with fuel.
This caused a lot of wear, on the first gen motors, especially upon startup.
It was common to see cylinder wear in the .007 to .0011 @ 100,000 miles on SBC's.
Many LS motors still have the cylinder hatch marks showing at this benchmark.

These are the same reasons the oil change interval has been extended from 3000 to 5000 miles.

With reasonable care I would expect 300k miles on a modern LS motor.
No the 5.3 are not more difficult to rebuild than the first gen 350 motor.
Replacing the head bolts, torque to yield bolts, adds to the cost but isn't more complicated.
The fact they last so much longer between rebuilds reduces the cost per mile.
Cheers.