Underpowered, because they are tempo losses. Generating shadow creep doesn’t impact the board immediately (except in the case of ooz).
Ooz is, imo, the only one that isn’t a tempo loss. (But don’t compare it to Amu. Amu is a neutral faction card and is inferior to all other faction 2 drop battle pets. It would be much more fair to compare Ooz to Fiz, Gro, etc.,).
Sphere of darkness is a tempo loss. It’s 1 mana, deal one damage. Playing this early game is a pretty weak play, and is almost redundant when you have your BBS up and can curve perfectly without it, so it takes up a card in your hand. It’s a good card, but loses tempo, compared to, say, bloodtear alchemist.
Comparing the stat totals of bonereaper and klaxon is ineffective (in general, this should be done with caution. Magmar’s 6/1 is much worse than a 2/3, for example). They serve significantly different purposes. Bonereaper is a high-health, provoke-stalling, swarm-countering minion. Klaxon is meant to do a lot of damage, with relatively weak health (Yes, 6 health is low for a 6-drop. A 6/6 body is underpowered for 6 mana. Tempo loss.)
I’ll mention Abyssal Crawler, too, since others are complaining about it. I’ll admit it’s strong, but it’s undeniably a tempo loss. When kept in the back, that shadow creep tile does nothing, and its body does nothing. Cass is spending 1 mana to do nothing. Even when she uses the body to trade or to contest a mana tile, it’s still inferior to bloodtear alchemist. Heck, it’s inferior to helm of mechaz0r, too, as a body.