I had one with a battery issue – it was dead out of the box, and after putting a new battery in, it would last less than a week where I’d only be using it for about an hour at a time.
However, I emailed Wahoo about the problem (it was about 10 months after I’d bought it as I’d not used it initially) and they immediately sent out a replacement without me having to return the faulty one, so top class service, though I suspect they knew they had a faulty batch.
The replacement works perfectly and the battery lasts fine, so I’d say don’t be put off by the reviews, as Wahoo do seem to sort out any issues!