7 sourced reports, with what to check before you buy. Each one links to the page it was reported on.
Costly or dangerous
Battery failure on GT 5 46mm and 2400 TL repair quote (TR)
A Turkish owner reports battery failure on the GT 5 46mm and a 2400 TL repair quote for the battery on a watch that cost 4000 TL.
What to check. If buying in TR, factor in that a battery replacement can cost over half the watch's price and that the owner reports Huawei charging for it even when the watch was never submerged.
GT 5 mainboard failure within first year, watch dead (TR)
A Turkish owner reports the GT 5 dying completely from a mainboard fault within its first year, requiring warranty service.
What to check. Total failure from a mainboard fault inside the first year is reported; the owner had to send it to an authorised service under warranty.
GT 5 overheated and never turned on again, no spare parts (Ukrainian owner)
A Ukrainian owner reports the GT 5 41mm suddenly overheating and never powering on again, with no repair parts available.
What to check. Sudden overheating followed by total failure is reported, and the owner says no spare parts exist for repair, so the watch is a total loss.
GT 5 stuck at 0% and stuck in reboot loop overnight (TR forum)
An owner reports the GT 5 waking dead at 0% and endlessly rebooting even on a fast-charge powerbank, concluding the battery died.
What to check. A GT 5 can fail to charge and loop-reboot overnight even when the battery was not low; the owner had to send it to service under warranty.
Official Huawei TR bulletin on Bluetooth connection drops on wearables incl. GT 5
Huawei's own Turkish support bulletin lists the GT 5 among applicable products and documents common Bluetooth connection-drop causes and fixes.
What to check. Connection drops are a recognised, documented issue on the GT 5; the bulletin's fixes are mostly environmental (distance, interference, app backgrounding) rather than hardware defects.
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.