In the classic mould of bad guys they get beaten by our heros, but unlike Stormtroopers they do pretty well against even the good guys, but regularly lose to the kids. I mean Dolohov kills Lupin, beats Moody, but he is Harrys punching bag and in the final battle he is stunned by…Parvati!!!
In HBP, while chasing after Snape, Harry dispatches what 3? 4? death eaters while running after Snape, and its like a throwaway moment, he is duelling and defeating wizards who have dispatched most of the Order earlier, but when he faces them its no big deal, indeed the main worry you have as a reader is not him being beaten, heaven forbid, but failing to catch Snape.
It is quite a training regime, you can beat immensely powerful wizards, but you will be consistatly beaten by teenagers…
No wonder Voldy’s orders were “he is mine”, think he was worried that Harry would kill all of them.