6 sourced reports, with what to check before you buy. Each one links to the page it was reported on.
Costly or dangerous
Xiaomi Mi Band 8 stops charging after 3 days of use
An owner reports the band worked for 3 days then stopped charging entirely.
What to check. Early charging failure means the band is unusable within days of purchase; a buyer should test charging immediately and keep proof for warranty.
Xiaomi Smart Band 8 case comes unglued and water kills it after ~1 year
An owner reports the case delaminated after a year and two weeks, water entered and the band died; another review on the same page says the sealing gasket leaked and the watch burned out.
What to check. Water-resistance appears to degrade over roughly a year; once the case unglues, water kills the band. A buyer should not rely on the 5 ATM rating past the first year.
Xiaomi Smart Band 8 stops accepting charge near empty battery after ~2 years
A Turkish owner reports the band stopped taking a charge at ~5% battery after two years; the cable and pogo pins were verified to deliver 5V, so the fault is internal to the band.
What to check. A band that stops charging near empty is effectively dead after warranty. The owner had to troubleshoot cable, contacts and power source before concluding the fault was internal. Cost is a full replacement.
Xiaomi Smart Band 8 unresponsive and drops tracking when paired with iPhone
A Reddit user reports the band becomes unresponsive, is hard to connect, and stops tracking mid-run when paired with an iPhone, while it worked with a Xiaomi phone.
What to check. Bluetooth reliability is worse with non-Xiaomi phones, especially iPhone. A buyer using an iPhone may face connection drops and lost workout tracking.
Xiaomi Smart Band 8 black screen and touchscreen errors acknowledged by manufacturer
Xiaomi's own FAQ states hardware failure can cause screen problems, and lists troubleshooting for black screen and touchscreen errors.
What to check. The manufacturer acknowledges hardware failure can cause display problems. This is troubleshooting guidance rather than an owner failure report, so it is lower confidence.
These reports come from the pages linked under each item: owner forums, repair guides and workshops. They are not a diagnosis of any one unit, and a careful check of the one you are buying still applies.
Text from the Kriko Smartwatches and Fitness Bands catalog, version 0.1.2, under the CC-BY-SA-4.0 license.