Yeah, they're still used cause they're good at plugging the gaps and filling out a raw quantity factor, but usually a well supplied and programmed squad of mechs, just using the cheapest tier of mook ones available, will probably kill double to triple their numbers in a lifetime, and that's if they're not just getting swarmed by greater numbers and are lucky in deployments.Otoh it is much faster to build robots then to train up new valks .
I wish the Herrscher of Void scene wasn't so spoiler filled, that probably had a decent showing of Honkai Beasts vs Mechs in the earlier part of it.
On the other hand a single squad of mediocre Valkyries will probably see dozens of deployments and hundreds of dead Honkai and Zombies, each, if they have a quiet career.
From cost effectiveness, Valks just beat mechs hands down.