Except, some of them (Soraka/Brand) have been borderline OP for ages. Brand because of his decent AP scaling, High base damages, Great AoE damage and tank shredding power has only ever been borderline because he's highly immobile, and has serious mana issues.
Brand's W is the only thing getting hit. This means that he'll have to sacrifice at least some of his build for mana/mana regen Instead of going straight for Magic Pen or AP like he has in the past.
Soraka with the buffs to support mana regen and the mastery changes, means that with less AP, she still heals for a metric ton and has massive Q sustain. In all honesty, mana nerfs are the best way to keep her insanely strong healing in place, while still offering some better counterplay. Her Q is now riskier for her to land max range, making it harder for her to sustain her healing. This accentuates her weaknesses without taking away her strength.
Swain has always been borderline OP. His big problem was again immobility. By accentuating his WEAKNESS, they make it so that opponents that can actually get up close to him, able to actually fight him. Even when they get up close though, he still has his great damage amp, slow and heal from his Q,E and R. Accentuating his weakness without hurting his strengths.
Riot has had the philosophy of "Clear strengths and weaknesses" and the latest set of buffs/nerfs to champions on the PBE are in line with that.
(I'm a soraka main and I honestly am not too hurt over the nerfs to her. I run Mana regen runes, so mana nerfs mean nothing to me)