For all sorts of diagrams, this software is awesome: yEd https://www.yworks.com/products/yed Favorite feature: automatically reorganize the chart according to your preferences/constraints (e.g. fewer line intersections, hub-spoke, hierarchical, etc)
yEd isn't exactly modern-looking though. I've recently used it on Ubuntu 16.04 but my colleague on Ubuntu 18.04 couldn't because the menu or window handles were cut off. As IntelliJ shows, using Java/Swing is no excuse for an app to look like a Windows 3.1 app migrated over to Windows95. But then UML is from a time and mindset when win32-like and Java-like OOP and Hungary notation were sold to people and I guess a ce…
We do have a more modern web-based graph editor: https://www.yworks.com/yed-live/ It may lack features you need, as the idea was a less cluttered and more modern UI, not to bring over everything and the kitchen sink.
As far as yEd Desktop goes, Java becomes an increasingly unattractive platform for desktop development, with Swing showing varying degrees of brokenness on different platforms and Java releases. The new hotness right now seems to be to either get working printing or working High-DPI support, but not both in the same Java release.
It's due for a minor UI overhaul, though, and this may come in the future. We probably cannot and won't completely restructure yEd's UI (such things also serve well to piss off long-time users).