Is Queen Medb worth leveling up?

She still has 5* stats and can nuke most male bosses.
Very good servant to have. Yes, Medb is worth it.

I would say, if you don't have a 5 star rider or if you have the resources and don't have another rider that you plan on leveling. Then sure.
I would rather level Ushiwakamaru. Shes a decent caster nuker. As for the anti male niche there are other servants that do it better than she does.
She's not at all a bad servant however. She rocked some of the Nerofest challenges.

Why not? Medb is at lowest tier because of other SSR riders performed great. But She still worked better than most 1-4 star riders in most circumstances.
Like if you make a rider critical team, or pair some male dps with her, she still worked good.