That change makes it useable for every class as well as makes it so the person playing it isn't guaranteed to get a card that's a part of their deck's strategy (and since secrets stay the color of the class they belong to the opponent always knows what cards it could be).
It's still all up side, but I don't see how it's more up side than it was before and thusly necessitates lowering its attack and hp (though if you want to argue it needs that nerf anyways, that's another thing).
1) Since Mad Scientist currently pulls from your deck, it means you need to have secrets to even get anything out of it in the first place, taking up deck slots.
2) When you have no Secrets, Mad Scientist is a vanilla 2/2 that in fringe cases buffs Undertaker
3) The lowest cost of a secret is 1 mana. Getting a random one means you're getting at least 1 mana worth of spells from your proposed Scientist, even if its Eye for an Eye. Otherwise, you have the potential to get a 3 mana spell out of your 2 mana 2/2
4) It would be really frustrating to play against, because you can never read the secret. Against Hunters, you know it's either Explosive, Freezing, or Snake. You'll make your decisions according to which one you want to trigger the least. Freeze Mage/Fatigue is probably Ice Block, Frost Armor. Echo Mage is Duplicate, Mirror Image. What are you going to do when the Handlock on the other side has a purple secret icon over their portrait? What play do you dare to make in that case? And every Secret Blizzard introduces from here on in will make your Mad Scientist more and more frustrating.
Secrets are generally narrow, unreliable cards. People aren't playing Mad Scientist to get their Secrets out. They're putting Secrets in their decks to make Mad Scientist a value play. And you want to introduce a class agnostic Mad Scientist that frees up 2-3 deck slots for other things!