I have a Samsung wireless charger (not the fast one) that I use for my S6 and it's really hit and miss as to wether it'll keep a reliable connection. I bought a much cheaper Chotech wireless charger from Amazon which I use at work and it doesn't miss a beat.
I'll be interested in hearing how reliable your fast wireless charger is over time, just to see if mine has picked up a fault or if it's common.
I have been using a third party wireless charger for a while and it works great.
I am interested in getting the fast version but don't know if it will justify the cost.