01 Placement control
Top bar only, or can you drop a timer inline on the product page, in the cart drawer, and on collection tiles? Inline product-page timers convert better than sitewide bars because they sit next to the buy button.
02 Honest expiry behaviour
Check what happens at zero: hide the timer, restart, or show a message. Apps that only offer an infinite loop per visitor are the ones that erode trust.
03 Scheduling & targeting
BFCM is the stress test. You want start/end datetimes in the store's timezone, plus targeting by product, collection, country and device.
04 Weight on the page
A timer is a few kilobytes of logic. If an app adds 100KB+ of JavaScript to every page for one bar, that is a tax on your Core Web Vitals.
05 What it loads, and where
A plugin that enqueues its script on every page of the site to run one timer on one product is a sitewide tax. Check whether assets load conditionally, and whether the timer renders as a block or shortcode you place rather than a sitewide injection.