Board Thread:Questions and Answers/@comment-32617252-20180806022624/@comment-29325824-20180806121229

As Nope2937 said, Tosen died because his Hollowfication wasn't nearly as stable as that of the Vizoreds. Urahara explained it as a process called "Soul Suicide", which he barely managed to prevent from happening to the Vizoreds.

So, no, he wasn't a perfect hybrid at all.