I don't think that Keldeo is underwhelming, I just think that people forgot how dangerous it is. Keldeo is checked by lots of Pokémon Pokémon (Primarina, Azumarill, Latias, Celebi, Slowbro, Slowking), but a Pokémon can check other Pokémon too. Keldeo has a really, really solid defensive typing, checking stupidly powerful attackers like Bisharp, Scizor, Krookodile, and others, like Water-types that are CM baits. Keldeo can run lots of sets too, I'll just name some of them:
- Choice Scarf
- Choice Specs
- SubCM
- Calm Mind + Waterium Z + Icy Wind
- Calm Mind + Normalium Z (Hyper Beam)
- Z-Rain Dance
- TauntCM
- Taunt + 3 Attacks
- RestTalk
All these sets are viable, perfectly viable, to be fair. The three most common sets are by far the Choice and SubCM, and they're already dangerous enough. Calm Mind sets with Z-Stones are amazing, not only because they are actually really hard to switch-in, but because bluffing with them is extremely easy. Z-Rain Dance is also a nice tool, since it allows Keldeo sweep the late game, outspeeding most Scarfers like Mienshao and super fast shit, like Mega Aero. Taunt sets are also surprisingly good, as they don't let Keldeo's checks work as well as they wanted to, like Mantine (TauntCM is actually a really good stallbreaker). Lastly, RestTalk allows Keldeo check what it checks, but with more reliability.
OK, after talking about Keldeo's sets, take a good look at your teams on Pokémon Showdown, and please, what team can handle all these sets? I mean, of course you can't cover all the sets, I'm just trying to show you how Keldeo is splashable and how it's dangerous. Keldeo is a beast when paired with Pokémon that provide Spikes and Pursuit support, since most of its checks have to be at full health to take its hits or are extremely vulnerable to Pursuit (cough Latias cough). I know why people are sleeping on it though; its checks are basically in every sort of team, however they're usually not checking only Keldeo, so they're overloaded by the rest of the dangerous attackers. Keldeo is not underwhelming, people are just sleeping on it.