Core Segwit — Thinking of upgrading? You need to read this!

http://www.wallstreettechnologist.com/2016/12/03/core-segwit-you-need-to-read-this/

Segwit cannot be rolled back because to unupgraded clients, a segwit txn looks to pay ANYONE (technically, anyone can spend the outputs). After activation, if segwit is rolled back via voluntary downgrade of a majority of miners software, then all funds locked in segwit outputs can be taken by unscrupulous miners. As more funds gets locked up in segwit outputs, the incentive for miners to collude to claim them grows. Compare this to a block limit increase hardfork, which can be rolled back by a block limit decreasing softfork.

Segwit doesn’t actually increase the blocksize, it just counts blocksize differently giving a discount for the segregated witness data.

Segwit doesn’t actually fix malleability bug or quadratic hashing, for outputs which are not segwit outputs. Yes, this means that as long as there are non-segwit outputs in the blockchain (for instance, long untouched coins like Satoshi Nakamoto’s) these problems will still be exploitable on the network.

Presently the danger of a 51% miner collusion is just the danger that txn can be censored, or that a miner can double spend their own txn. There is nothing that a 51% cartel can do to steal your bitcoins. But if everyone was using segwit, then that [stealing] actually becomes a reality.

Segwit grows technical debt. The idea of shoehorning the merkel root of the signatures into the coinbase message is a cludge made just so that segwit could be deployed as a soft-fork. How many kludges do we want to put into the Bitcoin base layer? Are we going to make soft-forks (and thus kludges) the normal practice?