Yes there is a cost if you don’t separate your concerns somehow.
Similar to others I also use the One Vault For Everything pattern with a Zettelkasten approach.
We should give it an acronym OVFE. 
I have evergreen notes in the root, but source notes and lit notes in subfolders with designators in the filename so I can easily see the difference between source note, lit note, and evergreen note.
I also have a folder for project-related notes. Currently these are notes regarding specific work projects not writing projects (as commonly described) - so these are “outside” the ZK structure in a sense but still useful to have in the vault.
Additionally I use the daily notes with calendar plugin. My daily notes almost 100% revolve around work commitments – meeting notes, action items, etc.
All of this is in one vault, two largely separate concerns in the same space but only very occasionally do they touch.
This is because I have only one brain. 