There are no known problems with Keyboard Maestro on macOS Sierra.
The current version (7.3) has no known problems on macOS Sierra.
If you download Keyboard Maestro, make sure to move it to the Applications folder or any other location after the download to disable Apple’s new Gatekeeper Path Randomization, otherwise version updates will fail (other than that, Keyboard Maestro does not care where it is run from).
In fact, all versions back to version 2.1.3 continue to work with macOS Sierra (with the exception of some visual artefacts and that they do not know about modern Accessibility Permission requirements).
It looks like currently the Send Mail Message action with and send now option is not working with Mail in Sierra - the message is left in the Drafts folder. Whether this is a bug in Mail or something I can work around I’m not currently sure about. It looks like it is possible to use AppleScript to send the message, so there should be some way to work around it hopefully, or at least you can use that method in the short term.
There is also a subtle change in WebKit in Sierra which stops the automatic reading/writing of variables in Custom HTML Prompt. See:
Version 7.3.1 resolves the Send Mail Message and Custom HTML Prompt issues.