ARB's are designed to be beaten on, E-lockers aren't.
"Destructive laboratory testing is carried out to ensure the strength of the Air Locker exceeds that of the axles or other driveline components. Environmental test cells subject prototype models to high (120ºC) and low (-24ºC) temperature running conditions, as well as harsh wheelspin engagement for cycle durations of more than 20,000 actuations. Finally, field-testing is carried out in the harshest environments imaginable."
http://www.arbusa.com/air_locker_design_engineering.htm
I don't think Toyota goes to that effort to make sure their E-lockers will hold up to that kind of abuse, the kind of abuse we like to put them through. In fact, they make sure you can't abuse it by forcing you to engage it only in 4-lo and under 5mph (little or no wheel spin).
I've also heard of several E-locker side-gear failures. I've got one broken one in my garage. Its not common, but it happens.
ARB leaks are primarily due to installer error. Mine has never leaked. I know others who have been running them for many years without leaks. The newer seal carrier designs are also set up so that it is more difficult for the installer to screw up the install as well.