I've seen them fail on other hikers as often as I've seen them work.
Must use lithium batteries, which are hard to find in small resupply towns. If alkaline batteries are used, you can usually expect to only treat 2-8 quarts, whereas Lithium may last up to 100 quarts.
Not as effective if water is cloudy or discolored.
Can leave the threads of the cap untreated.
They are heavier than my bleach and Sawyer inline filter method.
They are more expensive. Particularly when you include battery costs.