Here are some of my thoughts about why it is so terrible to me, which isn't representative of the whole 1K+ hour population:
The game has been around for 10 years there will be tons of hits, especially more recent weapon introductions but also a ton of misses or just introductory weapons. So the random will more likely hit those misses than the good ones but it isn't impossible to always hit bad though.
Getting the intrinstics that unlocked everything helps out with the additional offerings, which is why you see tons of complaining early on vs now since people now have those intrinstics and aren't complaining.
You also have tailored weapons builds built for specific frames yet using the random system can never get that. Say I have a kama prime and a kama and only forma the kama for a riven I would never be able to use the Kama unless I sell my kama prime.
Default loadouts are bad is a common argument I see but with decrees it gets better. However, decrees don't come by as often in the circuit which combined with the level scaling makes it worse as you go along instead of becoming godly like in Duviri.
Heck, there are even weapons that straight up cannot kill. I find this especially annoying vs orowyrm where you are already out of the melee slot but still getting options that can't even hit a final boss is a problem.
Time commitment, I haven't played circuit that much to know how bad this is since the gensis weapons aren't that interesting to me. But it is looking like a slog, if you are having to engage with the same content over and over again. I've dabbled in various content in Warframe and the general rule is the more I need to play it in order to get the rewards the more you come to resent it. Playing endless missions with limited missions and enemies is going to induce this.
Hidden mechanics, no official communication about invigoration weighting system leaves the doors (wide) open for whatever other "mechanics" DE forgot to tell us about the random offerings.
I love to experiment, find ways of utilizing a max reload speed build or a FR build or a parkour build but the circuit doesn't allow me to do that. Even if I get a setup that looks interesting there is just no incentive for me to back out and go mod everything only to go back in and complete a ROT C to never have that setup again.
I think the design is there with me actually going on Vauban or Mesa which I otherwise wouldn't do but the execution leaves a lot to be desired with me always having to pick weapons I sold off and the decrees not offering the intended boost that it is supposed to provide in the Circuit along with the being told hey time for your daily endless mission for the 20th time dear! So I don't think that it is wholly terrible siince you can still be given Mesa and cruise through but there is a lot of room for improvement, even beyond the random offerings, which it makes it hard without sacrificing too much.
