Is there some general trend for the operating pressure of reaction wheels? I can imagine low to no pressure has obvious advantages of low to no drag (more drag requiring more power, beyond that necessary to compensate for bearing friction), but sometimes lubricants, cooling, and electrical issues benefit from some atmosphere. It can help reduce migration of oil, help cool things, etc.
A poor analogy is a spinning hard disk drive. They usually require at least a half an atmosphere to maintaining proper head gap (the ALMA correlator (also All Systems Go for Highest Altitude Supercomputer) has no magnetic hard drives because of the altitude) so those would required sealed pressurization to work in low pressure or vacuum. Obviously reaction wheels have different constraints.
So I am wondering, do reaction wheels generally run in vacuum, or are they pressurized? Are they generally sealed or vented? I'm asking based on this comment.