Martin,
When you say a BSP service, I think you mean a generic service created in Tcode SICF.
So with such a SICF service, you do not have any rules on the request body or request URL, or request methods. But OData puts constraints on each of these and more. So OData is a standardisation layer. Any standardisation comes with its own set of advantages and disadvantages.
I would suggest reading here. Power of Standards: Why OData Shines in Time Cockpit - Time Cockpit
Regards
Krishna