I don't agree with the notion that you need to "understand" to be empathetic. If a friend tells me that they're going through a rough time I feel like it's disingenuous for me to tell them that I understand their situation, even if I myself have been through difficult periods. I can't and don't feel like I should give the impression I can understand the nuance of their situation. I've found it's much more productive to be supportive in situations like that by saying things like, "I know I can't understand fully what you're going through right now, but that sounds really hard and if I can help in any way please let me know" versus "Yeah, I understand what you're dealing with."

If empathy is putting yourself in another person's shoes and that's the understood general definition... I don't think it's genuine to say that, "Yes, in these few seconds/minutes/hours since you've told me your problem I have gone through the effort of visualizing life from your perspective and I now understand all of your issues." Even if "I understand" is communicated with good intentions I feel like it rarely looks to productive discussion.