Some of the impetus for mirrorless is because so many people want camcorders and mirrorless grew out of the video camera experience. Even with the move to mirrorless, these R series are still biased more towards a "still" photo camera with the ability to do some video work. If you need long video segments, you are crossing the line over to where you really need a camcorder with its active cooling system instead of a stills camera which is biased towards at least some level of weather resistance which isn't a good setup for rejecting internal heat to the outside environment.
A constantly active sensor, A/D converter, processor, and storage media are all creating a lot of heat and trying to go beyond 30 minutes puts an already seriously undersized cooling setup into the unworkable zone. The Canon 1 series have the best setup in the stills segment for cooling with a large body that dumps a lot of heat into the battery compartment where it can be more readily rejected but even these bodies aren't up to extended high bit rate shooting in high ambient temperature. Canon has warnings in the 1DX III manual that even the high speed memory card can become painfully hot to touch after an extended high bit rate video session and this shows that these components are being run at an unusually high temperature due to the inability to shed heat within a largely sealed body. This is where forced air cooled video oriented cameras are needed. I have three 1DX series bodies but rarely use them for anything more than a very short video "snippet". If I am shooting extensive video, I also carry my XF-400 and if I was planning to do cinema level video it would be with a C series body but the XF-400 with its integrated lens is perfect for sports.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
