He also is a limited songwriter. Ozzy didn't write the lyrics or the riffs in Sabbath - Butler did the lyrics and Iommi wrote the riffs. With Randy Rhoads he had a musical genius who wrote lots of tremendous songs and as good as Blizzard was, Diary was another step forward. Nothing touches the title track of Diary for combining classical with heavy metal at that time period. Later, Yngwie took it further on his Rising Force album and obviously raised the bar for technique by a tremendous margin despite Randy having good technique.
I don't think Diary had any weak songs whereas Blizzard had "No Bone Movies." Diary side one is unrivaled: Over The mountain, Flying High Again, You Can't Kill Rock 'n Roll, and Believer. That is just stunning.