[Riven Mods and all the problems with it.]

Of course getting drops in Warframe is random, almost every drop in this game is random to a certain extent so it would be weird if sorties didn't have a droptable as well.

Just do some sorties and eventually you'll get some rivens. Yes it is RNG involved when it comes to stats but I think only one or two of my 80 rivens went past 7 rerolls without passing the "not insane but better than a normal mod in this slot"-bar, most of them were playable or even very good already before that.

The price of Rivens is regulated by supply and demand only. On average there should be just as many Hind Rivens as Rubico rivens, it's just that nobody wants to play the Hind and many like to play the Rubico (looks better and is more powerful obviously) so they are willing to pay more for it to get one of these rare mods, if nobody would pay these prices they wouldn't be as high.

Also keep in mind that the WTS prices in tradechat are the prices of the sellers who try to set a price as high as possible so that they have room to go down with it, the actual selling price can be a lot lower or it can be a mix of other stuff + plat as well.

Riven.market may be misleading as well when it comes to the higher prices, look at the dates how long they are up already, if a riven sits there for a month it probably isn't worth the price the seller wants.

Regarding power/usefulness: Many rivens really upgrade some mediocre weapons to top-tier or offer stat combinations that are unaccessible otherwise and therefore increase the QoL of these weapons (punchthrough + X on pistol Rivens for example). My Dex Furis for example hit pretty hard now with my riven and seem to be pretty decent even at higher levels now, my dual cleavers Riven made them one of my best melee weapons while also being a good statstick for Gara/Khora/Atlas. I rediscovered quite a lot of weapons through rivens that I didn't bother to give them a revisit after my initial leveling (even weapons that have huge damage on paper but feel a bit awkward like the Exergis - my riven has reload speed on it so it doesn't necessarily deal more damage than one without, it just reloads faster). Rivens make so many weapons good that suffer from them having a low MR requirement.