Star guardian missions seem like a recipe for frustration.
Onslaught is pretty difficult, pretty team dependent, pretty imbalanced (adcs and supports > mages, certain level and enemy configurations are magnitudes more difficult than others), and pretty much not fun.
The missions set up seem to be introduced only 3 at a time so you can't easily double up on progress for a bunch of them, and many require multiple onslaught wins, which even if you get good at onslaught it still would feel repetitive to play through the same scenarios over and over again. Furthermore, half the missions (which are required if you want both rewards) both require a premade AND have very difficult restrictions, so you have to find four other people willing to put up with all of this bullshit to even qualify (my entire playgroup has already written off these missions as bullshit, so I would have zero help from any of them if I chose to pursue these missions.)
The mission rewards are a ward skin, which is OK but not particularly exciting, and a random champion permanent, which is probably the worst possible option for a reward.
Unlocking champions is more valuable to new players, but older players are more likely to put in the time to complete these missions. Assuming you go for the permanent because you either don't like the ward skin or you go through every single mission, then your reward varies wildly. You COULD get a 6300 champion which would be really cool (although given the time and effort it would take that also doesn't seem definitively 'worth'), a 4800 champion would be nice, 3150 would be satisfactory I guess. But what if you get a 1350 or 450 champion? There's a nonzero chance for less than 10% of the potential value. And what if it's a champion you already own on top of that? So all you'd get is a pathetic amount of blue essence with the value of like 2 games' worth of IP. What would a player feel like if that was their reward for organizing a 5 man group to grind one of the most repetitive and boring game modes Riot has come up with?
