Balance is very hard to achieve when you have a game with so many Troops, so many game mechanics, and an active community always trying to get the best teams out.
So yeah, every time you twink and fiddle with some mechanics, you risk to cross the fine line that will break the meta.
Remember when traits were introduced, a few troops could bypass armour on attack, like Skelton. Bypassing armour already existed with "true damage" spells, and skull production also existed. But still, just this little modification imploded the meta so bad that they had to fix it, and remove the trait.
I guess we are lucky to have a team of devs that actually work to keep the game healthy, and yet add more content.
So, back to DM and Devour.
Devour was not a problem, i think, with some ok Troops and weapons having lower then 30% chances (Sand Shark, for example). Great Maw is a strong Troop, but it's long to charge, and can be done only once.
With the introduction of 40%+ repeatable devours on good Troops with good traits (Kraken and Kerberos), we are currently walking a very fine line, as they are flooding the defensive meta pretty fast. Barrier will help. I think limiting copies of Troop would help too. We'll see. But these 2 troops popularity is increasing, because it just fills the gap, and makes it possible for a new, low level player to beat a 9000+ deck because he can just eat you up and steal the high stats of the opponent.
Deathmark was also ok, even weak. Casting something for 10% chance is a wasted turn. Adding it to some effect, like skull-attacks, is nice. Or on a weapon that does AoE damage and marks 2 Troops.
When mass DM got in (hello Death!), it started to be annoying. It's saving grace is that Death does not fill fast.
Death Knight broke the line. The spell is not only AoE, but close to unavoidable.
So now we get a small fix, we'll see how it works out.
It's very hard for the devs to foresee how the player base will embrace a Troop and use it, and how dominant it will become.
Finally, and most importantly, it is important to watch for player frustration. And, contrary to some beliefs, it does not always equal lost. I don't mind losing, and i win games that were frustrating.
So far, i would advance that, from my own reading of these forums, the main sources of "frustrations" are Famine, Death Knight, Kerberos, and Mab-Looping. (not my personal, i think Mab is funny. But then i play Oracle)
I think i lost my train of thought.