What is it that makes BIP149 safer to activate than BIP148?

Question

There are currently two BIPs floating to enable a "user activated softfork" of Segregated Witness.

BIP148 is meant to activate by flag day on August 1st 2017 whereas enforcing nodes stop accepting blocks that don't signal readiness for segwit.

BIP149 proposes to initiate a second activation process based on BIP8 once the current versionbit (BIP9) activation times out unsuccessfully. From my understanding, BIP8 is the same as BIP9 except that at the end of the activation cycle it goes into LOCKED_IN instead of timing out.

Isn't it just a semantic difference, whether a subset of all nodes defines the proposal as LOCKED_IN or the same subset of nodes outright rejects non-signalling blocks until the proposal goes to LOCKED_IN?

What makes BIP149 safer to activate?

I don't see a significant difference or that the former would decrease the chance of a chainsplit significantly except for the longer waiting time until it is activated.

Answer 1

BIP 149 is different in that it does not enforce orphaning of blocks that don't signal readiness for SegWit. Instead just the supporters enforce the rules on any blocks that contain SegWit data and allow regular blocks to be mixed into the chain. Thus, miners that don't change anything will build on the SegWit chain.

To split the network, miners would need to deliberate fork the chain, either by continuously rejecting every SegWit block and outpacing the BIP149 chain, or by mining a block with an invalid SegWit transaction that legacy nodes accept and SegWit nodes reject and building on top of that.