https://petertodd.org/2016/segwit-consensus-critical-code-review
Parts of his summary:
Segwit has a number of non-ideal warts at the consensus protocol level, mostly stuff another few months of development time and review could have ironed out (or resulted in a lot of arguments about bikesheds).
There’s a few “close calls” in the consensus-critical implementation that could lead to bugs later, but don’t appear to be actual issues right now.
Having said that, there doesn’t seem to be anything seriously wrong with the current consensus implementation or protocol design; at worst we’ve added a bit of technical debt, in exchange for getting critically needed functionality out the door.