I will say the iPhone 8's camera quality is comparable to a dslr. Alot of people will complain about the aperture/capture time but if you download FiLMiC Pro your going to be able to fine tune all your settings and get the best results.
This happened to me as well. I usually just close out the app and then re-open it and then the video plays. Pay attention to yur wifi connection as well, if you have a weak connection it wont play the video.