My big issues were the need to purge legacy IPTC data from my raw images, and the problem created by '.' in my keywords ( did not realize '.' was a default hierarchical separator) which caused Imatch to create keywords that it apparently couldn't write, and confounding several processes. I have documented this in the bug forum.
My conversion is done and I am halfway through purging embedded IPTC data from 30,000 raw files (doing 1000 at a time with ECP) , and doing all metadata write-backs left over from the conversion. I guess I'm confident enough now that this will workout to go ahead and upgrade.