To give some context in a militaristic perspective, Mechs are meant to be force multipliers. Similar to the Bradley and Abrams, solo they're vulnerable, but when supported by soldiers they provide excellent support.
Let's look at a Durand and Gygax's combat effectiveness against blobs, and terror spiders. I'm leaving xenos out of my discussion because I don't play them.
Durands are slow but well armored mechs and have a unique Defense Mode function. This means their best combat effectiveness is *supporting* a push, not frontlining it.
Gygax are fast, but not as well armored as a Durand, where they lack in defense they make up for in mobility. This type of mech can be used in front line as it as an ability to nope itself out of a bad situation provided it has an exit.
A blob is a stationary target, it's vulnerable to anything that is coordinated. Not to mention one of the greatest buffs blob has only works for energy based weapons, ballistics negate it entirely. I'd consider mechs a hard counter to blobs.
Terror Spiders are a threat to be feared however. They are mobile, they coordinate (or should), and in time have enough numbers to overwhelm most obstacles. They're at their weakest when they first start.
I don't believe mechs themselves need a nerf, like many things in the game, it takes skill and tactics to implement them properly.