In General limits when using Sync it states that
The Sync desktop app and web panel: Sync performance can get slower with more than 300,000 files in Sync. This is not a hard limit, and depends on the hardware specifications of the computer running the app (or web browser), as well as network speed, nested folder structure, and frequency of syncing activity. Many customers successfully sync more files without issue.
Also to note, there is a hard limit of 100,000 files per folder (but you can create an unlimited number of folders).
It seems that they are talking about a technical limitation, but that "this is not a hard limit".
Worst thing IMO is
What is the maximum transfer speed?
40 Megabits per second (5 Megabytes per second).
Nothing close to the astonishing Drive speed.
I've also seen box.com (https://www.box.com/pricing). They offer unlimited storage for $15/month or $180/year. The webpage has no info about how their unlimited plan works. rclone includes a box.com remote.