@ismael You certainly could implement a chat like feature using realm but it really depends on your use case because realm sync uses partitions to synchronize data between users - https://docs.mongodb.com/realm/sync/partitioning/
So if the chat rooms are limited and can be statically defined then you can share realms(partitions) between users which would replicate chat messages. For instance, an application where users are grouped into shared branch locations, and at those small branch locations users can chat with each other, individually or in groups.
The issue that can arise is if you need more complicated permissions within the branch or within chatrooms - the partitioning model is read or read/write per realm/partition per-user.
A chat application is pretty complicated in and of itself to implement which is why there are so many 3rd party providers that just do chat as a service.