In my understanding using HTTPS makes shared caching harder because caching on a router is not possible as information is encrypted. Therefore, requests can't be accessed as they can with HTTP. But does HTTPS also raise security concerns such as man-in-the-middle attacks?
HTTPS is a combination of HTTP and TLS. TLS is the the "S" in HTTPS.
And you are right. We can not simply use a caching proxy, unless we terminate the TLS on the proxy to get a plain text to be cached.
HTTPS was also designed to prevent the MitM attacks. If you do TLS right then MitM should not be possible.