Honestly, what bothers me the most about them is that for the same amount of money and work, you can clear 35s and have a heck of a lot more travel, less CV angle, much less weight and a lot more mechanical security.
You can absolutely do a drop bracket lift correctly. A nice set of front coilovers and rear shocks that are sized to eliminate the need for extensions. UCAs so you can make up for the extreme loss of castor. Differential drop to make up for some of that extreme CV angle. Maybe even LCAs?
But one should ask themselves when they get to that point: what's the point of the bracket? To get me another 3 inches at the sacrifice of added weight, worse geometry and lessened travel?
Why not get rid of the bracket, do a 1" body lift and do some trimming? You will end up with a lighter and safer vehicle that can clear the same size tires and probably have better clearance.
Then again, the majority of people looking at drop bracket lifts take their rigs off road a couple times a year...
I agree that some kits aren't necessarily set up perfectly. But the kits for the FJ give you longer much more heavy duty knuckles, you can buy extended travel coilovers that allow more flex than anything but an actual long travel kit, uca and lca geometry remains exactly the same unless you lift it further. You can lift just like stock suspension, so 3-4" more is feasible. They provide a front dif spacer to drop that back down like if you went with a toytec 3"+ or similar kit too. Cv axles remain at stock angles.
I have a 3" suspension and 2.5" body lift with 35's. I cut a ton to fit them. Someone running this kit could slap 35's on, trim the front liner and call it a day. Or get radflo extended travel coilovers and have more travel. They then could crank the coilover up and run 37's fairly easy. I was going to do that at first, but I'm going to hold out and sas personally.
For the rear, rough country gives you longer upper links offset for the lift and full lift coils, no blocks. Longer shocks are included and you get much more travel than stock and mildly lifted setups.
I guess the biggest point is this. These kits don't make anything weaker and provide extra strengthening parts even. Knuckles are included to keep all geometry stock. Spacers for the front dif as well to keep stock geometry. You don't need any more parts than what's in the kit or you can buy the coilovers and have much better ride, handling, and travel. For $1300 you can put 35's in it.
In my opinion I don't see the negatives. 3-4" suspension lift and a body lift is more expensive to get the same results. Bilsteins-$350
FJ coils front - ~$50
Rear lift coils-$200
Body lift-$150-$300 depending on size
Uca's - $250-$600 depending on brand
To run 35's you will need a minimum of 5" lift and a bunch of cutting. If you go cheap and get Bilsteins and lift coils you could achieve 3.5" of lift. Then you'd need a 1.5" body lift. For that much suspension lift you need uca's and dif drop. Already you are at $1100 minimum assuming eBay uca's. Then you still have stock, weak Knuckles, and have your rear arms at a high angle. Want to fix that? Buy aftermarket upper trailing arms for several hundred and the kit to gusset the spindles and you are ~$1500 minimum.
So really where is the debate? It's stronger, it keeps stock geometry, it's bolt on and off in the case of rough country, it's far cheaper, you get great travel all around and you can build on it. You could lift it more with coilovers or Bilsteins in the front and a spacer in the rear for up to 9" of lift easily. Sure, your center of gravity is higher. Offset rims and more careful driving, as anyone with a lifted rig should know regardless.
Negatives??? People that think they are cheap, poorly made, unsafe and haven't actually taken the time to look.
And if you look around you'll see DB lifted vehicles getting wheeled on a regular basis everywhere. Look at Keak's truck. He has 40's and wheels the shit out if that thing! Not exactly my style what he has going on, but it works.