A lot of people don't use it for two reasons. The first being they had some issues when they first came out. The design flaws have been fixed so the problems aren't there any more but once you get a reputation it's hard to shed it.
The second reason is that it's not a "true" locker. All they do is compress the clutch packs to keep things from spinning. Add horsepower and big tires and it doesn't hold as well, especially when one tire has great traction.
I think it's a great locker for the daily driver and casual wheeler. Their website said the Dana 44 locker should hold up to a 37" tire and that's as much as I'm going to run on the stock axles. If I go bigger I'm swapping in 60's.
Oh, and a couple things to note. Like any D44 locker, the JK D44 uses the standard D44 3.73 and down carrier for 4.10 and up gear sets. Also, the ECTED needs to have the holes for the ring gear bolts enlarged. Expect to spend a little time with a drill press when installing it.
2012 JK - 3.6/NSG370/NP241/D30/D44 - Grimm Jeeper
1986 J10 - 360/700R4/NP208/D44HD/D60FF - Truck Norris
1984 CJ7 - 5.3/4L60E/Atlas 4spd/D44/9" - Work In Progress
My gear ratio calculator