Disadvantages of using ASGI instead of WSGI
I didn't do any benchmarking but use both WSGI and ASGI in several project and didn't see any sufficient differences between their performance, so if the Django WSGI performance is acceptable for you then ASGI will work too.
For the REST + websockets API I used ASGI for both. There is no reason to use WSGI if you have ASGI enabled in your project (WSGI works over ASGI).
What is the explicit and clear disadvantages of using ASGI instead of WSGI for HTTP request handling in Django in general?
I know ASGI is for asynchronous tasks, but it can also handle synchronous HTTP requests via http.* channels. Is it slower than normal WSGI or is there any unsupported features comparing to WSGI?
One more, to provide both REST API and websocket handling in same project, which way do you prefer and why?

WSGI for REST + ASGI for websocket in different server instances
WSGI for REST + ASGI for websocket in same machine
ASGI for both