What are the risks with low disk space on a Mac?
There are a number of issues you could have with low disk space, but I wouldn't consider them high-risk. On a SuperUser post, the accepted answer cites a lot of information that is very old. The 10%-15% free space adage may even be out of date with APFS and modern SSDs. There is an older post on AskDifferent regarding the 10% free space limit with answer mentioning certain system process impacted by low disk space.

Typically, these are the main factors I've heard mentioned concerning having not enough free space:


macOS uses virtual memory. When more space is needed in RAM, macOS will swap some data from RAM to the hard disk. This is explained in an older Apple developer guide. If RAM is full and the disk is full, applications can crash.
Data that normally stays in purgeable storage, like caches and local Time Machine backups, will be unavailable for use. If you use the Store in iCloud feature, you could see many files unavailable locally that you would have to download again.


Anecdotally, I've run into these issues:


Large projects in Photoshop or Premiere couldn't be saved because there's not enough free space. This is risky since a crash could lead to losing data, even if I'm diligent at saving.
System updates unable to be installed due to lack of space.