FUD. Why should we want "behaves exactly as the browser does", when browsers (in fact, mostly Google's) are in fact turning against their users?
While browser vendors are wary of any sort of interception of HTTPS traffic
Especially Google's, because that's one of the ways you can still block ads and modify pages to have them displayed as the user(-agent) wants, while it continues to slowly remove abilities from browser extensions.
Similarly, the MITM might not support the most secure versions of protocols supported by the browser and the server (e.g. TLS/1.3)
AFAIK there is no general protocol insecurity with TLS <= 1.2, unlike SSL3. There's also no reason for MITM proxies to not start using 1.3 too once it becomes more common, which I'm pretty sure will happen.
this approach is generally preferable to MITM proxies.
...because it provides no way to modify the content, i.e. being able to do things like block ads, inject stylesheets, and generally behave as the user wants.
IMHO the whole "security" thing has turned into a power-grab for companies to enforce their control over users, which is the most disturbing part. We want security, but also control, which is not the "security" they want.
(This comment was posted from a browser that does not support even SSL 2.0, but is using TLS 1.2 via a proxy... and I know that 1.3 will be coming sometime in the future. When that happens, everything that goes through it can also start using 1.3.)
Edit: yay, instant downvotes! I hope you enjoy being herded by Google, because that's where the future is heading if we don't oppose.