Yes, if you do any clustering at all (vertical or horizontal) you cannot use our built in crontasks. Effectively you would need to create a custom controller that performs the logic you want and do a simple request (could be as simple as a standard GET request) to execute that controller.
Ideally you would want authentication on this and we do have a guide on our docs on how to implement an API token feature for the v3, whereas the v4 will have this natively built in.
The problem is that clustered Strapi instances are not aware of each other (stateless) so they are not “smart enough” to talk to each other to basically say “hey I’m node 5 and I’ll run the crontask, don’t worry about it!” and you would end up with all of your nodes firing the task at the same time.
There are ways to build the crontasks with some custom logic to make them aware (basically checking a custom entry in the database) but you start getting into deeply complex logic with that with very little benefit.