the one group of people the movie was slightly accurate in protraying was the canadians. they werent necessarily the hand picked best but they did have to pass an aptitude test to join.
the americans werent nearly as criminal and bad as the movie depicted. some of the american members were sprung from military prison (One being let out of fort Indiantown Gap as I recall) but on the whole, the main emphasis was for hard rock miners, mountainmen, hunters etc.
they did indeed depict the germans as bumbling idiots, I'm not sure the overall reason for it but it seems the biggest thing to keep in mind with this movie was their intent. it did capture the overall essence of the first special service force. when they were at anzio during ww2, one member was able to sneak into german positions and steal the ammunition out of the german ammo pouches; I dont know that there would have been a good way to depict both that kind of prowess and the professionalism of the germans however so they must have decided to stick to the forces story instead of the germans.
keep in mind too, in the final fight on monte la difensa, they depicted forcemen dying in really ridiculous fashion too; the perils of 1960's cinema

Yes and depicted the Germans as pussies who could not fight back worth sh!t. Horrible film in my opinion.
And no that doesn't mean I'm defending them, it means im defending the facts of war. Not just facts about the German army.

Seemed a bit prejudiced against the Germans though