1. Balancing is a pain. Tubliss is intended for off-road use, and the reason probably boils down to balancing the rim lock and heavy hardware for higher speeds. Provided you're not racing at high speeds, where tire heat may cause problems with the inner tube, there doesn't seem to be any reason why the Tubliss shouldn't be fine on-road except for balancing. I found that over 50km/hr, vibration on the front was a real issue (especially on long rides!) despite adding a whole lot of wheel balancing weights. Couldn't tell if the back wheel was also a problem. It just became too much for me, and no reasonable amount of Slime or weights made it tolerable.
4. Less weight with tubes. Although I didn't weigh the Tubliss vs. inner tube, clearly the combination of Tubliss tire, Tubliss inner tube, wheel weights, and bottle of slime per wheel exceeded the weight of a standard inner tube.
I never got the chance to try the Tubliss at low pressures, where they probably work well. I just didn't feel confident about risking tire or wheel damage at low pressure on some remote trail. Also, since most of my trail rides entail a long (50km) approach on paved roads, stopping to re-inflate afterwards would be necessary and time consuming.
KISS principle: tubes are simple to repair/replace. Combined with some Stans (not Slime!) for resistance to thorn punctures, they perform just fine for my type of 50/50 riding. In the end, I never really felt confident with the Tubliss system. However, I'd have no problem installing them on a pure trail bike.