Singing is not just about vocal ability though in terms of who has the biggest range or the most powerful voice. It’s about conveying the emotion of the song. Russell Allen leaves me utterly cold. Yes he can hit big notes and sing in tune but there’s no subtlety to him, it’s like being hit over the head with a sledgehammer (imo). JLB has a classy smoothness to his voice and does make me feel something, can bring me out in goosebumps at certain moments. Someone like a Mark Knopfler practically mumbles his way through songs but his voice moves me, it has a warmth to it that makes me feel good. Russell Allen sounds like every other Dio clone metal singer to me but lacks the subtlety that Dio had that made him great.
It’s all subjective of course, especially with singers (I love Axl Rose for example and others can’t even bear to listen to him!). If people like Russell, that’s awesome, I totally get it as the guy can undoubtedly sing, he just doesn’t do much for me. If he comes on in an Ayreon album, I don’t think he sounds bad, it’s perfectly fine, it just doesn’t elevate the material for me.