So yes, Romania was the most important ally, despite it didnt wanted and was forced in such position by USSR agression (combined with the pressure in the same time from Germany and its small allies around) and Romanian allies (France and UK) incapacity to fulfill the alliance treaties in 1940...
Udar, let's not play, again, the role of a victim in regards of Rumania.
Rumania was not 'forced', she willfully joined the anti-Soviet war and - alongside Finland - was the only nation Hitler actually counted on when penning the plan of 'Operation Barbarossa'.
First of all, its Romania, as you well know, but lets move on from this little problem of yours
Until 1940 Romania was allied with France and UK. In 1940 however, we found caught alone between USSR (who just signed the secret Ribbentrop-Molotov pact) and Germany who supported its puppets around (Bulgaria and especially Hungary - see the Viena diktat).
France was already occupied and UK barelly escaped having luck that it didnt have a land border with Germany, but it was out of question that they was able to do anything on the continent anymore.
So Romania was alone facing Red Army, Weh
rmacht and Hungarian and Bulgarian armies (nastier situation then Poland for ex). It was a decision to fight against all these or to agree to much of their demands on the idea to preserve as much as possible of the army and teritory for future action to recover the losses when will be possible.
From what I read, the General Staff considered that the Army will be able to resist just a week against a war on all borders facing the enemies mentioned above. We didnt had even enough ammo at that time, let alone the lack of tanks and the limited number of aircrafts and heavy artilery. Basically Romania wake up too late and paid the price for the mess of the politics, even corruption and lack of rearming preparations during the interwar period
One of those moments arrived with Barbarossa operation, when we joined Germany to recover our teritories lost to USSR.
This wouldnt have happened if USSR didnt take those from us in the first place.
About Hitler counting just on Romania and Finland, it was a normal choice, as those was the only countries with an enough competent and even big enough army (I may add even secret services in some cases here) to help in a campaign against USSR.
Actually I think Romanians was the only ones who had German troops under command, up to armies level (under command of General Antonescu in 1941 and in 1944 I think another German army was under command of General Petre Dumitrescu if I am not mistake).
Our problem was lack of modern weapons, lack of tanks and heavy artilery especially, and in significant parts lack of training adapted to new conditions from WW 2. The last one improved during the war, and units as Mountain Hunters or Cavalry was quite an elite ones.
But in the large and open steppes you can't fight against huge masses of tanks and infantry without to have as well enough tanks and heavy guns.
At Stalingrad for example we had just one AT gun at few km of front, which was way too overstreched anyway. Germans refused as well our proposition to allow us to eliminate the Soviet bridghead over Volga, as the orders was to concentrate just on Stalingrad, and dissmised the conclusions that Soviets will attack to encircle the Axis troops.
Even so the first waves of Soviet attacks (especially the infatry ones) was repulsed despite the huge artillery preparation used before, but it was impossible to fight with one AT gun and rifles against masses of T-34 and KV tanks.
German Panzer group under command of General Heim, which was in reserve and supposedly ready to act as back up didnt do scratch. In the same time most of our troops was overhelmed and fight to the death (I think there was even Generals dying in bayonet fights).
Heim was probably too shocked to react and his group intervention didnt come. I think he was even court martialed and arrested after that, as Marshall Antonescu was furious about the outcome.