The only time I find it ok, is when a reaction is used when a conversation is at a natural end, or when its followed up with a reply.
I find it borderline rude when people use it and then don't bother to respond properly with a reply. Example, I send a photo to a friend 'I saw this and thought of you!' I get a ''person' reacted 'heart' to ...'' and no reply. It basically makes me think they don't actually want to speak to you or make conversation, it actually makes me feel a bit rejected, like I'd probably feel the same if they just didn't reply and left me on read.
It was actually better when people replied/responded with an emoji than them clicking the 'reaction' button.