I run campaigns where the party is split all the time.
On one hand, my feeling is that if there are N players, each player gets 1/N of the playtime. That remains true whether the characters are all in the same little room, or in different solar systems. (Not that I’ve ever done one with different solar systems!)
On another hand, if the party is split, the characters may not be able to interact with each other; they may only be able to interact with NPCs. That may or may not be a minus. It does mean that for that span of time, they have the GM’s undivided attention, and they get to have their characters take action without competing for the spotlight. That may be a plus for players who are less assertive.
On a third hand, it’s possible to have subgroups of players interacting with each other, but not with the entire group.
Probably the most important thing, though, for me, is that it lets me develop the action on several parallel tracks, with cross-cutting. That can be a great way to build tension. Not having that available as a tool would be really limiting.
Now, in practice, my campaigns range from some where the PCs are a team and spend a large part of their time together, on shared missions, to one or two where the PCs are only all in the same place in the final couple of sessions. They have different dramatic goals; for example, if I want a campaign where the PCs are in conflict much of the time, letting them pursue victory in separate plotlines works pretty well.