The '86-'88 2.9l had the weaker heads, prone to cracking if overheated, '89-'92 heads were redesigned and stronger.
All 2.9l were prone to leaky valve covers, but that could be any Ford, lol.
Also a bit of a design fault was the oil supply for the valve train, it's pressure was dependent on the cam bearings wear, so as the miles went up the valve train got noisier, never seen a failure based on this, just noisy.
That being said there are alot of 2.9l's still on the road, it has reasonable power for a small V6.
The 2.9l block was the base for the 4.0l, which means a 4.0l will just bolt in, to replace a 2.9l