There are several problems with COSMOS, circle, and Data Center missions.
They’re not advertised. If I hadn’t read DMC’s Repair Plan I wouldn’t have known about the COSMOS missions to begin with. Back when the CQ was a thing I sure as heck knew about the SOE arc and all the Epic Arcs but there’s sod all pointing me at these. At least the new standings UI does hint at the epic arcs but IMO it’s now more obscure than the CQ ads.
COSMOS missions have a faction standings requirement. There are a few Caldari missions that you can start at 3.0. Amarr’s lowest is 4.0. The Epic Arcs let you use corporation standings as well as faction standings. Some players may find that much easier to bang out.
Some are broken. According to my research the Amarr circle agents haven’t given any standings boost since 2012. And before you ask, there are numerous claims of having reported this issue.
They’re one and done. That’s a levelling mission. Like it or not, there are people that demand repeatable PVE content as their end-game.
The way some of these are chained together if you decline one, that’s it you’re locked out of the rest of the arc. Not sure what happens if you have one on offer and you don’t get to it before it expires. When I ran some recently I made very sure to complete everything.
Their payout’s nothing to write home about. I can make more mining in high-sec on a per hour basis.
Finally, Standings. Derived standings are a neat mechanic… for a single player game (mmm, Torment). They’re a terrible MMO mechanic and even worse here it’s well enough hidden that the first hint many players get is when they’re popped by the faction police.
Fixing that can be insanely unfun and harm your ability to run L4’s in your normal missioning pocket.