A lot depends on how you set it up and what you do with it....
PLEASE note all comments are JMHO
Cooling is always an issue, the short/wide radiator is just not the most effecient design. Later models with the 3 core radiators do better, four core does not help.
Always carry a spare Crank Position Sensor, they go bad and you are stuck. OEM replacement only!! I had 3 bad ones in a row from Auto Zone.
Handling is VERY dependant on lift!! Anything over 3" do a long arm. 5"+ a long arm is mandatory!! I prefer a radius arm style long arm. The "mising link"/only one arm on the passenger side creates odd handling/loading, and chatter under hard braking. Radias arms on BOTH sides with rubber bushing will allow MORE than enough flex.
I went through 6-7 pairs of coils and 4-5 different leaf setups. My opinions: RE leafs are the best ride/flex/value and for coils I liked the ZJ frt coils in the XJ. The ZJ ones are ussually a little higher spring rate, expect about 3/4 more lift in the XJ then they give in a ZJ. Also perfer RE here. The Tera coils seemed too soft, but their ZJ ones were ok.
RE, Clayton, TNT, all of good long arm setups.
I would highly suggest the uni-frame stiffeners!! After 12+ years of wheeling an XJ it turns into a wet noodle!!!!
I built ours and many others, please ask away....I know NAXJA and JU can be unfriendly at times....
Always glad to help....