In case a UASF is deployed and activated by exchanges there could be a stand-off situation in which miners mine on a different chain than exchanges. This would look like a sudden and long-lasting reduction of transaction capacity to exchanges. The situation appears to be very similar to a hard-fork in this regard.
If 75% of miners reject the UASF the UASF-nodes would perceive a reduction of transaction capacity by 4x (to 25%) for 8 weeks until the difficulty adjusts (2 weeks times 4).
Are exchanges capable and willing to go though that phase? It seems the damage would be considerable (like it would be in a similar hard-fork situation).
Exchanges and users would suffer from higher fees and from not being able to transact with the majority portion of the ecosystem.
This question seems relevant to assess the viability of a UASF.
