For a big snake, it's no problem. The little guys though can't get any purchase on the bottom and if they can't pull themselves out by their chins, they get tired and drown.
I lost a some baby rubber boas this way long ago. It was a really cool bowl, but had sloped sides and I came in one morning and found 2 dead babies in the bowl. They had drowned. I've only used vertical sided since then for the babies.