I do not support the BIP148 UASF for some of the same reasons that I do support segwit: Bitcoin is valuable in part because it has high security and stability, segwit was carefully designed to support and amplify that engineering integrity that people can count on now and into the future. I do not feel the the approach proposed in BIP148 really measures up to the standard set by segwit itself, or the existing best practices in protocol development in this community. The primary flaw in BIP148 is that by forcing the activation of the existing (non-UASF segwit) nodes it almost guarantees at a minor level of disruption.
Segwit was carefully engineered so that older unmodified miners could continue operating _completely_ without interruption after segwit activates. Older nodes will not include segwit spends, and so their blocks will not be invalid even if they do not have segwit support. They can upgrade to it on their own schedule. The only risk non-participating miners take after segwit activation is that if someone else mines an invalid block they would extend it, a risk many miners already frequently take with spy-mining. I do not think it is a horrible proposal: it is better engineered than many things that many altcoins do, but just not up to our normal standards. I respect the motivations of the authors of BIP 148. If your goal is the fastest possible segwit activation then it is very useful to exploit the >80% of existing nodes that already support the original version of segwit.
But the fastest support should not be our goal, as a community-- there is always some reckless altcoin or centralized system that can support something faster than we can-- trying to match that would only erode our distinguishing value in being well engineered and stable. "First do no harm." We should use the least disruptive mechanisms available, and the BIP148 proposal does not meet that test. To hear some people-- non-developers on reddit and such-- a few even see the forced orphaning of 148 as a virtue, that it's punitive for misbehaving miners. I could not not disagree with that perspective any more strongly.
Of course, I do not oppose the general concept of a UASF but _generally_ a soft-fork (of any kind) does not need to risk disruption of mining, just as segwit's activation does not. UASF are the original kind of soft-fork and were the only kind of fork practiced by Satoshi. P2SH was activated based on a date, and all prior ones were based on times or heights. We introduced miner based activation as part of a process of making Bitcoin more stable in the common case where the ecosystem is all in harmony. It's kind of weird to see UASF portrayed as something new. It's important the users not be at the mercy of any one part of the ecosystem to the extent that we can avoid it-- be it developers, exchanges, chat forums, or mining hardware makers.
Ultimately the rules of Bitcoin work because they're enforced by the users collectively-- that is what makes Bitcoin Bitcoin, it's what makes it something people can count on: the rules aren't easy to just change. There have been some other UASF proposals that avoid the forced disruption-- by just defining a new witness bit and allowing non-upgraded-to-uasf miners and nodes to continue as non-upgraded, I think they are vastly superior. They would be slower to deploy, but I do not think that is a flaw. We should have patience. Bitcoin is a system that should last for all ages and power mankind for a long time-- ten years from now a couple years of dispute will seem like nothing. But the reputation we earn for stability and integrity, for being a system of money people can count on will mean everything. If these discussions come up, they'll come up in the form of reminding people that Bitcoin isn't easily changed at a whim, even when the whims are obviously good, and how that protects it from being managed like all the competing systems of money that the world used to use were managed. :)
So have patience, don't take short cuts. Segwit is a good improvement and we should respect it by knowing that it's good enough to wait for, and for however its activated to be done the best way we know how.