I have found that large partial classes declared across many physical source files creates lots of issues with intellisense throughput. At 50+ partials, you are looking at solid 10+ second delays in the IDE between when you typed something naughty and when you are informed of it. This behavior has been replicated across many different computers and versions of visual studio, so I think it's just a fundamental limitation.

We have some solid use cases for free floating functions (e.g. business rules that need to be able to arbitrarily invoke each other) which we ultimately decided to scope to one public function per class. This allowed for some other interesting advantages in terms of consolidating tightly-coupled business logic that was formerly spread across the codebase (e.g. items used only by the one function - mappers, request/response models, private utility methods, etc). We even explicitly duplicated out some logic in order to enable completely isolated verticals of BL.