When gear diffs first came out in rc, they weren't fluid filled. They were always unlimited slip. While this was easier to drive than no diff at all, it created some problems on certain surfaces but was inherently reliable. Sometime around 1980, Cecil Schumacher adapted an old go-cart style diff to an rc car. This was the first ball diff in rc. He tried to patent it but the idea wasn't original. The ball diff was superior to the non fluid filled gear diff. Towards the end of the 80's we started filling in the Kyosho style gear diffs with grease. This was a great setup on dirt ovals but the grease would still leak from the outdrives over time. It got messy. I still have an RC10 belt drive that has this very setup, grease and all.
Keep in mind that up until slippers became widely used, we ran the ball diff loose on purpose to double as a slipper. Yes we rebuilt them quite often! It was that technique that ultimately led to the creation of the dedicated slipper. The nice thing about a ball diff was that it could be quite loose and still send more power to the wheel with lower grip than a gear diff could. It could have much less resistance and still not diff out as badly. Tracks were much looser then too. A downside to the ball diff however was that how loose we could run it was affected by how powerful of a motor we ran. A mod motor needed to run a tighter diff so it wouldn't slip. As grip came up this problem became more apparent. It's an issue we still have with them and is a current downside. You don't have that problem with a gear diff. The difference between the 80's and now is that we don't want a ball diff to slip. With the advent of the fluid filled gear diff, we could run a looser diff and not have it slip. The roles reversed. However we can still get a diffing out situation.
As a general guideline I'd say to run a ball diff in lower grip and a gear diff on higher grip. I do hate adjusting ball diffs though. As they wear they change and it is nearly impossible to set them up the same way twice. Removing a ball cup and adjusting them through an outdrive is also a pain. In 4wd setting up ball diffs well can be a nightmare. I personally would prefer gear diffs only in 4wd but my B44.1 is stuck with only a ball diff right now. Removing and taking apart a gear diff to change fluid isn't fun either but once you get it where you want it, you can repeat it with another one and for the most part it doesn't change much. Temperature will affect it but the main concern is from outside temperature changes between winter and summer and this is fairly easy to account for.
Neither system is perfect but neither is always better than the other. I wish they'd go back to the JRX2 LRM gearbox style. The diff was in the middle. You removed a dust cap to insert the allen wrench to adjust the diff. It was simple. No removing axles.