I am a canadian citizen. My husband is a canadian resisdent. About 20 yrs ago he fled from Cuba. He use to work for Castro... so you may say he is a deserter.
Do you think it is safe to go and visit after all this time even if he became first a Canadian citizen?

Depends on what he did and what level he was. If he was a high level staff member who gave out secrets to other goverments when he left, most likely he is not safe completely where he is living today.

If he was a low level worker it may depend if the government filed charges on him there, when he left, So official channels would be needed to see if there was legal issues first.