Is Medea worth the investment?

Yes, she's the go-to ST Caster for Masters w/o Sanzang. She also has the niche of being able to dispel enemy buffs (for enemies that like to spam evade or guts) on demand.

Her NP has a lower multiplier compared to other ST Arts NP. This is because what she lacks in damage, she compensates for with her ability to do back-to-back NPs.

Her NP is pretty bland at first glance. It's essentially damage + dispel. However, she also gains NP gauge refund after cast the higher her Overcharge is. Couple that with her 1st skill already giving her 100+% gauge on demand at sufficient level and that she's a Caster with 3 Arts cards + Territory Creation passive, she boasts a pretty spammable NP. She also gets her third skill once you do her Strengthening, which gives a targetable clear and NP gain buff.

Edit: Her NP gets a sizeable damage boost after her interlude so get her to the necessary bond level to see higher numbers. Also, do Daily FP summons and get her to NP5.

Just know she is the only dmg caster for f2p. The other offense casters for this year are sanzang and illya and maybe caster marie, but they are all rare.

NP1, no interludes. that part may be your problem. NP1 will always be wet-noodle damage. get her to NP2-5 for proper damage and her interlude will make it even better. max fou, max NP post interlude you can expect about ~72,687 damage a hit without buffs, with waver it's around ~94,493 and as a caster with all the arts cards in the world she's gonna be using that near every second turn, this is why she's the go-to caster. the buff clear and debuff clear are both nice additions too, as a free to play however you should probs give her a Mozart to support her with his 1st skill and enemy debuff noble phantasm and friend Waver.

Same with me bro, I have her at NP4 and she hits Assassins like a straw, I am utterly disappointed in her, she is the only top tier f2p servant that I think is very overrated and bland, if not for her debuff removal she is pretty useless, her survivability is terrible, so is her damage, I prefer Anderson over her.
Maybe you can spam her NP every turn but it hardly matters if all it is going to do is compete with a single buster hit of a berserker, a lot of people like her but all I can say is that she is an utter disappointment for the so called offensive caster.