Imgur now blocks several VPNs and have issues loading embedded previews in several fediverse platforms. So instead of using imgur, you could use one of the following alternatives for uploading your images.
https://postimages.org/
https://imgbox.com/
https://imgbb.com/
https://www.imagebam.com/
Images could eat up the server resources of your instance. Using a third-party service reduces the burden on them.
Anything that you don’t host yourself are vulnerable to takedowns. But as someone who has been using postimages.org for many years now, I have never had any such issue with them, and haven’t heard of anyone else facing them as well. The other three services I linked also have a good reputation as reliable services.
As an instance admin I gladly host user files.
One thing to take into account is that images posted by an instanceA user on an instanceB community will still be hosted on instance A.
So as long as an instance doesn’t host more users than it can handle it should be fine.
From what I understand reading this thread, if instanceA goes down, any images hosted there are lost, while the comments will still exist because they’re federated. You’re only shifting the responsibility of hosting the image from a site like imgur to the home instance of the poster.
I guess it comes down to if you’re concerned about how long your home instance is going to be around for, use an external host, or see if/when account migrations are added if images move too (although they would also have to fix the src for wherever the image is now being hosted)
A downside to hosting images externally is that these image hosts can go down before the Lemmy instance does, leaving many posts without context. One should keep this in mind when choosing where to upload their images.