Different classes are different, I agree. But it should not be directly stronger than the other.
Thinking about it, Siphon Life is stronger than Plague Lord, but it's not strictly stronger. 3dmg+3heal is probably stronger than 3atk/3 armor, but it not increase your maximum "health pool". Moreover, they come with a 6mana and 10mana DK, which are also different.
The point is, there can be cards that stronger than the others; there can also be class cards that strictly stronger than theirs basic/classic counterparts, they are okay. Having a class card that is directly hit anothet class card in the face is not okay, no matter what.
I am pretty sure I missed a few, but the point is, cards from different classes exhibit different power level, depending on whether it plays to the class's strength or not. That's the half the point of Class Identity
Also, on the topic that Warlock spells being generally weaker, Cho'gall is an amazing legendary that does not see any play, simply because majority of Warlock spells are crap. Bloodbloom doesn't see play either until Darkest Hour comes out, because, once again, warlock spells are mostly crap.
This card is very nice for arena. For only 2 mana dealing 3 damage+ healing? This is a upgraded version of "drain soul" which i liked. Its also a common so for sure atleast 2 of them and you are fine. I rate it 4/5
Because warlock have the best hero power in the game by a large margin. Blizzard have always had to be very careful about the power level of all warlock class cards because by default warlock's hero power puts them at an immediate advantage over any other opponent from another class.
Another potential reason is that healing is unusually good in warlock because so many of their cards (and also their relatively very powerful hero power) are balanced against HP loss. Many pros and streamers have said that whenever warlock gets strong healing tools, they dominate the meta (or some have said in the past that if warlock gets strong healing in future, they will almost certainly be a top meta contender). We can see this has been arguably proven correct with the reign of cubelock while Kobolds and Catacombs was in standard: Blizzard eventually nerfed Dark Pact from 8 healing to 4 healing.
Putting aside the comparison to Siphon Life, this card is a very strong early game tool for Priest against aggro (looking at your Underbelly Angler), and as the livestream suggested it is a perfect follow up to a Crystallizer turn.