The app probably is rubbish. I think I did well for it not to be another source of anxiety when I couldn’t hear it!
I know there are various things said about dopplers too. The way one of the MWs explained it was that if someone was passed out on the floor, you wouldn’t think they were fine just, because they had a heartbeat... and same is true of baby... which is why movements are so important later.
Also I’ve read things about the ultrasound waves of using it too often but there is divided opinion on lots of things as obviously it’s not ethical to test safety of various things during pregnancy.
I think using a once in a while is probably fine