Is it safe to run do-release-upgrade on a cloud VM?

Yes

I just did this to about 110 EC2 instances. I actually did it from Ubuntu 14.04 through 16.04 and up to 18.04 using an automated the process for that many instances. It turns out it is possible to automate do-release-upgrade

If you want to be really safe, take a snapshot of the root disk before going forward.

Make a clone of your instance or spin up a test instance and do the upgrade. There are a lot of changes in Ubuntu from 14.04 to 18.04, a lot. A number of things broke, and SystemD changes a lot of default behavior.

But there are quite a bit fewer changes from 17.10 to 18.04, so I expect you to have a pretty reasonable time of it.