Actually, from what I've read/researched, the older 4.0's have slightly stronger blocks due to a higher nickel content.
AFAIK the Renix is a good system, with some minor faults. They have a problem with the way things are grounded, but this can be corrected with cleaning the grounds and/or augmenting them with heavier duty ground wires. The cylinder head and the intake manifold aren't as efficient as the later (1991+) models, and the amount of vacuum lines is another potential trouble area. There isn't as much aftermarket support for the older engines either, so it can be tricky to upgrade them.
On the plus side, they are pretty rugged. A lot of folks claim to have limped their Renix-based Jeeps along with sensor faults that would have stopped a later, more complicated version in it's tracks. I know that the OBD I and II versions can be tricky to mess with because the computers don't like the changes; but they can be reprogrammed, unlike a Renix model.
One of the better things I've heard, is that you can diagnose things on the Renix system with a good multi-meter. No confusing, or misleading error messages like the OBD units can spit out.
I honestly think if you have some electrical know-how, and are willing to keep it tuned, adjusted, and maintained; the Renix should be just fine.
Plus, if you hate it, you can buy a rusted-out POS XJ with a good drivetrain, and swap it ALL over.