Well in 1943, the Germans had annexed parts of Italy, and created a puppet state called the Italian Socialist Republic. They were fighting the Guerrillas in Northern Italy and the Allied forces in South-Central Italy. Croatia also was a German puppet state that had to be defended by German divisions (Although they weren't German), most notable being the 7. SS-Gebirgs-Division and the 13. SS-Gebirgs-Division, not forgetting that the entire German 2. Panzerarmee was stationed in nearby Serbia. Once the Soviets got close enough to the Romanian border in late 1943, German troops were required there aswell, same with Hungary since neither of those two country's armies could withstand the Soviets at any rate, i mean just take a read from what troops from the Grossdeutschland-Divison had to say about the Romanians:
Spoiler
(1st Battalion), Pz.Gr.Regt. GD and its Romanian allies occupied a well wired-in and T-Mine-strewn line running to the right of 3. SS-Pz.Div. "Totenkopf's" positions. The Soviets advanced in great waves, their tanks and infantry mixed together. Moving much faster than the Germans had anticipated, they overwhelmed...(the 1st Company), wiping it out to the last man. While (the) temporary battalion commander...attempted to halt and repulse the Soviets with what was left of his battalion, the Romanians to the left of (the 2nd Company) cowered in their trenches like "herrings", as (the) company commander...later recounted. Indeed, the Romanians seemed to have reached an unofficial cease fire with the enemy.
After '43, the Romanians hardly fought the Soviets to any degree, they mostly surrendered. Although i will say the Hungarians fought much better than the Romanians, they still needed alot of assistance from the Germans. The Slovaks hardly ever helped, their tiny army was only of use in the Invasion of Poland in '39, and later went on to due a National Revolt in '45. The Bulgarians surrendered to the Soviets as soon as Romania had fallen, they didn't even fire a shot.
All in all, i would honestly say the Germans had little use of it's allies other than Hungary and Romania (Romania being it's oil and position as a springboard into southern USSR). Germany would have been better off mounting two puppet governments in the two previous ones instead of making them allies under their own independent governments.
Italy literally fucked up everything for the Germans, that may be harsh but honestly i have a hard time explaining it better. The Italian soldiers may have been much braver than the Romanians, but the High Command was full of incompetent jerk offs who were lead by a flamboyant tough guy who knew nothing about strategy and the army (Mussolini).
Germany was able to mobilise up to 20 million soldiers during the war, the problems they had however was distributing them. Instead of reinforcing existing units, Hitler would often divert the recruits to form brand new units instead, which gave headaches to the high command that had to move around 'divisions' that were the sizes of regiments.