My understanding is that Access mdb's are OK for up to 25-30 simultaneous users in general, although these numbers may have been a theoretical max and apply to heavy intranet or internet usage. Sure, that is not 'enterprise' level, but it is fine for any electrical design project that I've ever heard of. Lot's of apps use .mdb's internally, including at least one (if not all) of the major competitors to ACADE, and an app that I've used a lot called Parts and Vendors, a multi-user BOM database application. "Enterprise" would generally refer to 100's of simultaneous users.
That said, your milage will certainly vary depending on how efficiently and intelligently the application uses the .mdb.
Incidentally, I doubt you will ever see any ECAD systems use SQL Server any time soon. That would like swatting a fly with a sledge-hammer, even if it is a really light, well-balanced, super-duper, multi-user one. That just means the expense of licensing it for use in your app would be greater than your profit margin. Of course, that might not be a problem with ACADE. :0