The most frustrating thing for me has been poor quality aftermarket parts:
Water pumps that fail in six months.
Distributor caps that lead to phantom misfires (on one I determined that the area holding the carbon rod was too long and actually contacting the rotor, causing the mechanical advance to stick)
Brake pistons that were too small :Wow1:
Again, this has been with aftermarket bits. Rover original parts haven't been a problem (other than price $$$ and availability), but as has been noted that's become a single source/limited supply issue. It reminds me of the situation that frustrated me years ago with my MG (talk about being born to tinker) -- you'd have an original 20 year old part finally fail, buy what was available only to have that fail in a few months.
With regards to the belt/sensor comments, if one is up for enough tinkering then injection system, engine front area or even full engine swaps are no big deal on these trucks. My '87 is a mix and match of various years parts, though I haven't gone serpentine on it yet.