I don't think it is a fair question. Some think tedious and difficult to play is most fun because only the really dedicated will play. So complicated equals serious.
I disagree. I feel a serious game need not be overly complicated. Competition is serious. If the definition that it is totally a dice and card games which can be fun even if passed out drunk- then I would say that "Lying dice ancients" could be fun ancient rules- but maybe not serious.
A lot of people used to dismiss WAB because it was a dicey game that looked to them like a "skirmish" game.
That labeling itself was a buzz word for lack of "seriousness", i.e. dismissive.
But WAB could be as serious or as simple as players wanted. As simple as a DBA two foot table "battle" with 36 figures, or as complex as an ultra detailed DBMM giant recreation of a historical battle with detailed OOB's.
Both could be fun, both could have uncomplicated rules (if they were written in intelligible language)- both could be serious and competitive.
WAB could be serious and competitive as well as being simply a skirmish game :
The biggest problem with ancient rules is that everybody wants the latest hotness to cover their own personal army and that always leads systems to get more and more watered down to accommodate or elevate obvious disparities between time periods that stretch for thousands of years. To me the only "serious" ancient game is one between actual possible combatants.
Hyborean Islander Fyrds vs. Macedonian Successor Kyrenians with chariots, is just a fantasy game. A fun game probably- but not one to take seriously.