Of course tastes are tastes and anyone can dislike Deris' voice or the songs written with him as much as they like, but I feel that more than anyone else Deris has a thankless job and is a very underrated singer.
First of all, he saved Helloween in 1994, if he hadn't joined, the band probably wouldn't have gone on. Yeah, call it PR but the band was really in a down mode at the end of 1993 and their mindset was "If we can't get someone we can trust in the band there's no point in going on".
Then he's a singer with a specific voice, like it or hate it but he's no Kiske clone or anyone's clone, he has his own voice and style and - for better or worse - you can't find anyone else like him in the scene.
Another important factor is that he's a damn good songwriter, so he brings definitively a lot to the table creatively.
And finally, and this can't be stressed enough, he's a great frontman. Always entertaining and able to work up a crowd. I'd daresay that, if you don't consider the gods of the genre like Bruce Dickinson, he's actually THE best frontman in metal.
So, as I said like it or hate it, but Deris saved Helloween and contributed strongly to the band ever since he joined. And what does he get for his troubles? complaints for the way he sings song written for a teenager with an inhuman voice.
I bet he's the first one out of all the fans who's glad that Kiske's back, so he can finally stop to sing Eagle Fly Free and get shit for it