I'm still deep in IMF revisit. IMHO if you haven't got the europeans blus with the DTS-HD MA tracks, theses are the prefered upgrade. But be aware the sources are the same as the old discs, it's not like the Die Hard remaster for example, which uses a different scan from prior releases. Do not expect Matrix like new eyes on those films.
Expect the same looking prints, now upgraded to 4K HDR.
The reviewer from AVSForum received confirmation from Paramount that the first three movies were sourced from new 4k scans of the film elements.
The old Blu-ray edition of M:I-2 used an ancient DVD-era master smothered in DNR and edge enhancement. It looked awful. Every review I've read of the UHD other than yours claims that the UHD is a huge improvement.