I can confirm the problems stems from Photos having trouble importing photos/videos directly from the iPhone. I found a workaround:
- use Image Capture to grab all the photos you need from the iPhone and import them to a local folder on your mac
- drag all photos from the local folder back into Photos library
- the photos will be added to your library as usual
Before I found this workaround, I used Image Capture to preview all the photos from my iPhone and when I tried to drag these iPhone photos to Photos, guess what, Photos crashes. I can recreate this crash again and again.
This bug is ridiculous given importing photos is such a fundamental function of Photos. So the team at Apple, please look into this without further delay. BTW my iPhone X and 2013 MacBook Air are all on the latest OS and my Photos library is stored on an external HDD.