faster wear of front end parts.
easier to bend the c"s or tubes(if used hard).
Its a matter of leverage.
once the tires start to stick out it will toss alot of everything everwhere. Im at 3.75 back space, about as wide as you can get. helps off road in off camber. makes it harder to push through some rock areas that are narrow.
I would expand on the first half of his post. Proper bolt on spacers add no more leverage than wheels with the same end backspacing. Any addition stress the drivetrain would experience is no different between spacers or new wheels.
The others are all right that if you widen the stance you will get more debris as the fenders aren't perfectly plumb. With a 4door the rear bottom hinge will catch tons of dirt and rocks.
Not nitpicking on 222Doc in particular, but I see the misconception that spacers are somehow inherently more damaging to a vehicle than wheels that do the same job. Properly
installed bolt on
spacers are perfectly safe. Those non bolted spacers that just gap your wheel and rotor are extremely dangerous and should never be used.
I recently sold a 2005 TJ with the spidertrax spacers on it. I used that jeep for both daily driving, 2/3 of which were highway, and offroading through mud, sand, and washed out dirt trails. The spacers never failed, nor did any of the drivetrain components. I've seen numerous people on this forum in the TJ forum running for over 10 years.
The biggest downside to spacers? tire shops who are deathly afraid of them.