Pros and cons of disabling the default Microsoft key:

(Assuming you have secure boot enabled, and want the security that comes from that)

pros:

• You control your own key and have full choice over what software can start up on your computer, software cannot be approved by anybody else.
• Your secure boot security model is not vulnerable to the risk of booting 3rd party software with known security vulnerabilities.
• Sophisticated attackers with physical access to your computer cannot carry out an evil maid attack on your computer and convince it to trick you or steal your data.

cons:

• You need to have software installed to manage the key. There is software available for Ubuntu and NixOS.
• There are many buggy UEFI implementations out there that require the Microsoft key to load built-in oproms during standard boot, potentially bricking your computer.
• Software that gains root access to your computer could steal your signing key, potentially negating the benefits of secure boot against non-evil maid attacks.

That said, I use it my own signing keys, since I have a Framework AMD laptop that does not require MS keys to boot, and I consider evil maid attacks (going through customs) as a higher risk than accidentally running malware.