I searched a little and found the following blog, which claims, that there are issues with it, when you do heavy data transfers, for example via many rsyncs at the same time:
I suppose that’s a good point. With multiple large simultaneous flows, multiple TCP connections probably will be better, unless SSH goes to all of the trouble to reimplement all of TCP’s nicer features. And at that point you should just be using a real VPN anyway.
I wonder if ssh shouldn’t have a sensible default like ~/.ssh/control/%C for ControlPath, so that you could just turn on ControlMaster and have it just work. Then TRAMP could set the ControlMaster option on the command line when it runs ssh. At least then people wouldn’t have to mess with their SSH config, and they wouldn’t have to consider whether it will break something else.
I searched a little and found the following blog, which claims, that there are issues with it, when you do heavy data transfers, for example via many rsyncs at the same time:
https://www.anchor.com.au/blog/2010/02/ssh-controlmaster-the...
I did not test it myself. This would seem like an appropriate reason to not make it the default.