Using a doppler because your are curious, it to aid bonding and to get more engaged with the pregnancy= great (IMO)
Using a doppler because you'd like extra reassurance= dangerous (IMO).
If you can use a doppler, but find a heartbeat, shrug and thin it's probably just hiding and go and make dinner - then get one. If you'd be panicking, that's reassurance you're after.
I used one with both pregnancies, very much for bonding and because it's cool. I was and to let mum MIL hear that heartbeat of her grandchildren on Christmas day. She cried.
It's not done mystical art - it takes a little practice but not much. As long as you've looked into it, so you can properly differentiate between the different sounds I found it pretty easy. They sound very different. If you do it frequently, you also find out where baby likes to hang out which makes it easier. I had to direct my midwife to where she was at my 16w appointment as she couldn't find her until I spoke up.
But even if you are good at it, them be times in the early months (less at 20w+) where the baby will be in an awkward spot. Can you handle that without panic?
I used mine a few times a week from about 11/12 weeks, and took some recordings of them etc. It was great for bonding.
I don't want to see them banned, but I do with that women would be more realistic with themselves about their level of worry and self control before they buy one.