When designing new components for a game there is a real struggle to balance new effects that are weird and new. There are good reasons to underpower them deliberately at the outset just in case something comes up that you aren't prepared for and they end up being a lot better than anticipated. People react much better to buffs for underused effects than they do nerfs to their favourite stuff. However, when you devote a bunch of coding time to a new thing it is a bit of a waste to make it so weak that nobody uses it and your effort is pointless. For example:
The Spelleater is neat but awful. She gets rid of your current Hero Power and you get your opponent's Hero Power instead. That is a cool ability but since you don't know what your opponent's Power will be and it rates to be about as good as yours this is usually a disadvantage, not a benefit. The problem with the card is that there is a basic card that is a 6/7 for 6 mana, which is similar to the Spelleater but with 2 more toughness and it is hard to imagine when the Spelleater would even be as good as the old one, much less better.
Even if you managed to engineer a situation where you knew that your opponent had a Power you wanted (which seems impossible) and that your Power was weak if you cast the Spelleater on turn 6 you are still near the end of the game and are barely even going to get any use out of the Power you steal. This special ability is so bad that the Spelleater shouldn't be *worse* than the base unit, it should be flat out better! If this lady were a 6/8 instead of 6/5 it would be fine because her special ability is random and usually not great so her being slightly better than a card that is good in Arena but useless in constructed is no problem.
So Blizzard spent time and development resources to create a new and interesting effect that will pretty much never see play. No one is going to build a deck around it because they have no idea what it will do. No one is going to run it in some obscure combo. It is just a waste of a slot, which isn't a big deal if the effect in question is super simple, but it is a silly thing to do when you have to put a bunch of effort into making it.
Diablo 3 has a huge number of similar sorts of issues. Many of the most interesting and intensive effects that legendary items have are junk and never get used. A classic example of this was Grin Reaper which summoned several copies of you to cast your spells. It takes a bunch of coding to set up new units with new AIs that have all kinds of things they can do, and originally the Grin Reaper was just trash. You had the choice of a few different helmets that doubled your damage in various ways, or you could put on the Grin Reaper and have a couple of dorks that flailed away uselessly for you. No choice at all, that. However, this one has a happy ending because Blizzard finally made the Grin Reaper powerful enough that it has a solid place in a number of nuker builds and the resources they put into making it function are now actually being worthwhile.
I do respect that as a game maker you don't want new and weird stuff to come in totally over the top, but in the case of the Spelleater in particular I think blizzard really messed up. In Diablo 3 people expect items to get changed and buffed all the time so if you bone it up you can always buff later. That isn't the way hearthstone works though so I suspect the Spelleater is going to be a total waste of virtual cardboard forever.