utf8mb4 is generally available for production use since we introduced 7.5. Of course, you should test it with your application before rolling it out into production.
Disk space use should be unchanged because utf8 encoding is a variable-width encoding. So turning on mb4 instead of mb3 should take the same amount of space for the same data.
I don’t expect any significant impact on load speed & query speed for identical data.
Regarding any restrictions on function use, all string functions are supported on utf8mb4 data. Of course, this is a brand new release and a new capability, so testing your application in advance before going into production is even more important than with a mature release. If any issues are found, we’ll work promptly to resolve them.