I have a charge 2 which I wore continuously (day and night except for showering) for about 18 months. I took it off one day and noticed the skin underneath was going like translucent. Definitely not an allergy, more like the skin was degrading. I stopped wearing the fitbit and after about 3 weeks my skin seemed to return to normal. Needless to say I won't be using it or any other fitbit products (possible any other similar products from other companies) anytime soon.
I also didn't like that I couldn't get access to my time-series data. It seems I can only access one day's worth of information at a time. I really wanted to build up a picture over time so this was no way to download this from the device, especially considering all the data is sitting right there on the device.