I feel like this depends on your definition of “OP” if you believe hive scum and psyker are “OP” then yeah skitarii is with them, it is definitely a step down but it is competitive with them in some ways.
I could be wrong but I feel like this will be another case of people sleeping on things, like currently you see most experienced players running around with chordclaw, and chordclaw is a very solid high damage but still support focused build. But the DPS is not quite there.
My personal pick for most “OP” build for the class is Arc Rifle + Advanced Combat Doctrines (link). So far in my over 40 auric games that is the build that is consistently doing near or at psyker damage. Its not at where gunscum was on launch (something people definitely slept on for a while) but I have still topped damage literally every single time I have played the build, usually by a wide margin and pretty consistently by close to double the next player. And its ammo efficiency makes me think this performance would carry over to Havoc but Im not sure if I will be playing Havoc currently so thats not for me to discover (unless I can get some friends together I just really cannot be bothered with making parties in game lol).
That build is slightly over powered if you ask me and would push the class into OP. BUT the class is ACTUALLY a glass cannon and not a pseudo glass cannon like Psyker or hive scum where their mobility and toughness regen just negates their low toughness pool. When you build Skitarii for max DPS it is actually pretty squishy, and for that reason I think it is a step down from psyker/hive scum, so A+ tier. I really think the class could be a strong contender for #3 strongest class. Im sure many people will say that makes it OP, but Im just ranking it comparing it to what is currently in the game, and to me “OP” would mean “better then everything else” like hive scum is/was.