I downloaded the Acorn TV app onto my Amazon Fire Stick, and it just constantly tells me that I'm not connected to the internet. Meanwhile, I can watch Stan and Prime on the Fire Stick without any problems. Do not recommend this app at all.
UPDATE: May 2020, and Acorn TV still has constant buffering. I tried it on Chromecast from my laptop, and it's just as bad. I like the shows on offer, but the constant buffering is so annoying. Netflix, Stan and Prime all run without any problems. I checked my internet speed, and that is also fine. Sorry Acorn, I like the shows you have on offer, but the constant buffering makes it unwatchable. Unfortunately I paid for a year's subscription in advance, but I will definitely not be renewing when it runs out.