In humid weather, the caseless cartridges would bulge from the moisture and either get smushed or fail to chamber. The magazines full of caseless ammo could not be submerged in water for any length of time.
The biggest problem was out of battery detonations. Designers of caseless cartridges seem to fail to take into account how much transient chamber heat is removed from the chamber by the ejected brass case. With caseless rifles, the heat stays put.
The G-11's weren't exploding after extended rapid fire, though... the cartridges just went up in a big bright fizz like a 'safe and sane' firework when the shellac'ed cartridge met the red hot chamber walls. They had a lot of 'runaway guns', I heard.
I believe that Hk has completely given up on exploring caseless ammunition firearms. So has everyone else, it seems.