Hi Andrew,
I've been involved in Mongo projects and seen a few from a distance. From what I have seen, you must make sure you meet all the requirements for using MongoMK[0] and you must have a network/hardware available as specified in[0],key considerations such as RAM, Network speed(no cross region), disk IO, these are critical, as the recommendations have come from various engineers who have been on projects that ran into issues, so treat the recommendations as gospel.
On the MongoDB side, you really do have to get people trained up or find people with experience of MongoDB, in order to monitor, debug, backup etc
From various performance tests at a particular client, we saw performance was on par with CRX2(slower than TarMK), but as the load grew, performance was consistent for MongoMK, whereas it deteriorated for TarMK.
Regards,
Opkar
[0]https://docs.adobe.com/docs/en/aem/6-2/deploy/platform/aem-with-mongodb.html
[1]https://docs.adobe.com/docs/en/aem/6-2/deploy/platform/aem-with-mongodb.html#Hardware%20Considerations