I've only heard Operation Mindcrime from Queensryche but it was incredibly meh
I wouldn't necessarily be as negative as Zantera, but I'm not at all shocked that someone thinks it's meh. For its reputation as one of the prog metal albums (and it certainly is; I'm not denying that), a ton of it is mostly just straightforward NWOBHM. Mainly just the concept parts and Suite Sister Mary are not. I was definitely underwhelmed the first time I heard it, and while I've come to enjoy it, I can see someone still having that underwhelmed opinion.
I also think Queensr˙che is mostly going to appeal to people who like that style of metal, while Metallica will appeal to a much broader range of music fans. I like but not love that style of metal, so it makes a fair bit of sense that I like but do not love
Operation: Mindcrime.
I see Metallica as one of those standout bands that transcends genre in terms of appeal, while Queensr˙che really doesn't do that in the same way. I'm not surprised to see anyone, really, being a fan of Metallica. I'm also not surprised to see someone like Zantera, who doesn't really seem to be that into the whole 80s style of metal, not be that impressed with O:M. By a similar token, I don't typically like thrash, but I love Metallica. And
Rust In Peace, which also has genre-defying appeal in my estimation.