As author of OpenRTS, I would say that classic RTS like starcraft are hard to make. It requires complex (but well documented) algorithms and tons of adjustments. Here are the trickiest parts imo :
- Faction AI (economic and strategic decisions)
- Unit AI (finite state machine/behavior trees, path finding)
- Balancing of factions is just... a mess
That said, none of it is engine related, so it's acheivable with jMonkey as well as with any other engine.
Note that the balancing and faction AI can be simplified. A scripted AI is often sufficient (like in starcraft solo campains) and balancing is mandatory only in a multiplayer game, with few factions.