I've got 2 external HDDs in ZFS mirror, cheap consumer disks. One of them is perfectly fine, second one started having occasional read or write errors, which is not great. With regular weekly scrub I hadn't had any data issues over last 3 years, and I'm pretty happy with the setup.
But it's a hit or miss with consumer external HDDs - before that, I used a single external HDD for backups at a time (100-200GB writes a week) and I lost data from 2 cheap external HDDs over span of 3 years.