Yes, I have the same experience playing with the County of Flanders. The mercenary tab is available but no units to recruit / hire.

However if your general is in your vassals territory you can (sometimes) recruit a special unit. It changes every turn and it wil not always be available.

The recruitable units depends on the vassal so if your vassal is Scottish, you can recruit scottisch longbowmen one turn and hobbillars on another turn.

Personally I haven't seen or experienced any late game units or features. Maybe this will be released on a later time but I presume it involves cannons, guns and late-period armour.