Is it safe to define a very high ulimit for a process?

I currently have the following entry in /etc/security/limits.d/90-nproc.conf:

Mongod soft nproc 64000

I need to massively increase the number of – let's say – 999999. In normal situations, only a few thousand files are likely to be viewed per day.

Are there negative impacts on this situation?

Is it detrimental to system performance to have a large number of open but largely unused file descriptors?