I've functionally mass-subscribed to every community that overlaps with my primary interests regardless of which instance it is on and make use of the feature to view submissions from subscribed communities.

The fragmentation is frustrating because it makes individual communities seem less populated than the topic actually implies. For example, there are multiple large Games communities across the biggest instances, but as they are not on the same instance, people are likely to participate in a subset of all of the available communities. This generally reduces the volume of participation in any one community, even if the volume across all those communities summed up is very substantial.

A "multireddit" at the community level would be quite nice (rather than the process of subscribing to a large number of communities and using the "subscribed" feed).