In my old house I have a fuse box, it is not a consumer unit, it was made before the type testing came out, it now has MCB's fitted there are two fuse boxes both fed from RCD's which are in a box with built in DIN rail and until the metal CU came in it complied with all regulations other than being type tested, at the point where the RCD's were added there were not any CU's with twin RCD's.
Only reason why Wilex MCB's were fitted was they were only ones that would easy fit a Wilex board, however with DIN rail mounted MCB's with a copper bus bar feeding them I have seen where massive strain has been exerted as the terminals were tightened as one make lifted the terminal to clamp and another screwed down. Strain on a RCD I have found caused it to fail to meet the tripping requirements, with an MCB they are not tested after fitting so it could result in the devices being out of spec.
"Could" is the problem, we simply don't know, no manufacturer tests a board with other manufacturers components, even 2 mm difference in bus bar alinement's could produce a strain which we are unaware of. If using loops of flex between each MCB then there would not be the strain but getting flex able to carry 100 amp is also a problem.
There will be situations where one can get around the problems, but then the CU is no longer a CU it is just a distribution unit, as to how that would effect with Part P is for courts to decide.