Yes, in this v2.0 version the early Hungarian units disappear (go home) just before the first winter comes, whatever happens, as historically. (Maybe they knew something? Or they were more realistic given that they also lacked winter equipment and clothes and did not want to fight under these terms. Unfortunately the German units did not have to luxury to do the same...)
But on Uhu's request I changed this in the latest versions of the mod: since BE v2.13 the Hungarians can be taken home by train where they would spend the winter months and then can be taken back to the fronline the next spring with some additional new units. This way they can retain their experience. If, however, the player does not move them home by the deadline they disappear anyway as in BE v2.0 and then replaced by inexperienced units the next year.
It depends on the strategic situation. If things do not go well most Axis nation will quit the war at some point, disbanding all their units. They may even turn against Germany as it happened historically. In most cases it was unexpected and caused great problems to the German war effort. In order to avoid it the player has to do better than the Germans did historically.
As for German units, they do not disappear on their own if I am right. The only exception may be the two Hs 126 recon aircraft units, but these will be replaced by a better Fw 189 recon plane, and the Hs 126 units cannot get replacements and cannot be upgraded before that so the player cannot lose with this. But again, in the next version of the mod the new Fw 189 only appears if at least one Hs 126 is still alive at that point so that both cannot be wasted. In this v2.0 version though the new recon plane will appear whether or not any of the two Hs 126 units is alive at that point.
It is indeed important, but this is a historical mod and historically the general quality of the Lufwaffe decreased gradually. Mainly due to the shortage of aviation fuel which forced the Gemans to cut the hours of flight training. It resulted in having less and less well trained / experienced new replacement pilots taking the place of their fallen comrades.
This chart shows how the hours of flight training changed with the major powers during the war:
http://www.ibiblio.org/hyperwar/AAF/AAF ... fe-LXX.jpg
While between 1939-42 new German and British fighter pilots had about the same hours of flight training, after that the Germans had less and less and the British and Americans had more and more. From mid 1944 a British or American fighter pilot cadet had 3-4 times more hours of flight training before entering frontline service than that of a German. The reduce in flight hours in operational aircraft for the German pilots was especially significant. (For example a fighter pilot cadet mostly flew in trainer planes and only had a few hours in Bf 109 before being sent to the front. Basically he only learned how to take off and land in a Bf 109 and they expected him to learn the rest at the operational unit.) Therfore later in the war the pilots of the Allies were much better trained and in general were more experienced than the average German pilot. To some extent it is reflected in this mod as well.
For more:
Strategy for Defeat
The Luftwaffe
1933-1945
http://www.ibiblio.org/hyperwar/AAF/AAF-Luftwaffe/
