What's wrong with a mix of BG and Contactum though? Surely both of them do the same thing.
The DIN rail is a German standard, but where the operation lever comes through the lid, and the height and method of clamping onto the bus bar is not standard.
There have been consumer unit fires, in the main caused by a poor connection to the bus bar, the MCB's where the terminal was not forced open by the terminal screw allowing the bus bar tag to go wrong side of terminal clamp, and the different heights or bus bar increases the chance of an error being made.
Also the consumer unit type testing means only items recommended by the manufacturer can be fitted, otherwise is ceases to be a consumer unit, it is just a distribution unit.
So that's what wrong with mixing makes, however for an EICR we would need to code C3, and recommend correct MCB's fitted, but if they do fit OK then could not be a code C2 potentially dangerous, however since the bus bar is held firm by the other MCB's and the MCB is held firm by the DIN rail it is nearly impossible to tell if the fit OK.
And I know I was called to a new house with a complaint the phones were acting up in the house, but were OK with old house, so sent to reassure the new owner there was no problem, however I was getting odd readings with the loop impedance tester, so opened the CU, and found the bus bar to MCB screws loose.
I reported what I had found and the electrician who fitted it was questioned, and it seems the CU had come populated, so he had never checked those screws, seems the supplier had just slotted the MCB's in so easy to carry, and the manufacturer did say all screws should be checked, however it shows how easy it is to make an error when the DIN rail is holding the MCB firm.
But the second electrician was not it seems doing an EICR, so commenting on another electricians work it seems likely he was looking for more work for himself.