I think the only time I ever truly hated secrets was while running Ramp Druid for awhile (and old combo Druid to some extent) while mech mage with mirror entity was one of the most popular decks. Mage getting a free secret and having Belcher, Druid of the Claw, Ancient of War, Sylvannas in hand was absolutely a pain in the ass. typically the best play was charge DoC and essentially lose my turn.
I don't play Druid anymore, but they have a lot more ways to play around Mirror Entity now. Tempo mage is still a big pain in the ass for Druid, though and secrets don't help.
I've also had situations against Hunter where they get running Freeze Traps with Bow other removal and are able to basically keep you locked down until Turn 6 and by then it is too late.
All that said, I agree with the others and only truly believe that secrets with mad scientist are overpowered. If played from hand you have to look at them as a 1 for 1 card advantage and potentially a tempo gain or game winning play (with Ice Block) that could happen with any card played at the right time.