Of course. But the baseline stack of hardware and software for RS232 is (or at least can be) significantly simpler. USB and Ethernet are nightmares by comparison, and that's before considering the additional protocols that invariably sit between those low-level transports and your application.

For a project idea I've been considering how to attach a sensitive (but powerful) machine to the network with the smallest possible interface (in terms of code exposure). RS232 is probably the simplest yet still functional without getting into niche hardware standards.

On balance I think USB might be worse than Ethernet[2] or even Ethernet+TCP/IP just because typical network stacks have probably seen more scrutiny than USB stacks. But at the end of the day I know I'm going to want to use something like seL4 to protect the application from the transport stack(s). And even then you still have to worry about the hardware. There's a real dilemma in choosing the simplest hardware that is still viable in terms of performance, support (including ease of porting or writing drivers), and long-term availability.

[1] Assuming a simple ethernet controller with all the fancy features disabled or, ideally, not even present.