I've used to play the non-steam version before, so my playtime here is inaccurate.
MM: DoC struggles with several issues that make it hard to enjoy in the long run.
Firstly, progression is very slow. Rewards for a single match are negligible and you're only rewarded with a reasonable amount of gold (basic currency) when you level up. The problem is, after the first few levels it will take you around ten hours of playtime to get another one. Some mechanics are in place to make the grind less monotone, but they have their own share of issues.
There's a PvE mode available in the form of campaigns. Each campaign consists of 5 missions that, upon being completed with one of the six available factions for the first time grant a significant amount of gold plus some experience points. Clearing the entire campaign with a faction also grants a free booster pack. The problem is, those free boosters are largely useless, since following an overhaul in 2014 most cards from older sets can no longer be used in ranked play. The gold rewards for campaigns also happen to have a nasty incident in their early history. Two weeks after being introduced those rewards were increased by a factor of five and this increase was not retroactively applied to players who already cleared the campaigns. Since the increased rewards were ridiculous - clearing 2 missions took next to no time and granted as much as a single level up normally does - pretty much all the players who weren't already veterans found themselves in a situation where it would simply be easier for them to start the game anew (which isn't allowed), even if they already spent money on it.
The previously mentioned overhaul also made it impossible to gain seals (premium currency) without paying money for the game, even though previously they were received upon leveling up and as a reward for 6 consecutive days of playing. Since every new card pack is only available for seals for the first few weeks, this also means that paying players now have an actual advantage over non paying ones.
There's also a lot of balance issues. Unless you manage to build a decent deck, which involves lots of grinding or spending several AAA titles worth in money on card packs then you'll struggle even against mediocre necropolis players, mostly because dark magic school is broken to the point where it enforces the meta (not worth it to use any creature costing more than 4, because otherwise casting Soulreaver or Death Seal will remove it for a lower cost, while casting Wither will make it useless). Necropolis isn't exactly overpowered though: it just isn't really possible to counter if you lack cards, so for the first ~40 levels or something around ten times that amount in dollars/euros.
If you bothered to read this wall of text, you might've noticed that most problems are nailed down to the controversial overhaul, that:
1. added PvE campaigns and then screwed over everyone who finished them
2. made over half of the cards in the game useless by excluding them from ranked play or the so-called 'standard format'
3. removed all the free ways of gaining premium currency, except for some achievements that are mostly hard or even impossible to get without paying for the now obsolete card sets in the first place.
Point #2 effectively negatively affected paying players by making some neat decks that require a select pool of cards useless and wasting their money in the process, point #3 made the game feel like a 'pay to win' for free players, while point #1 was a bad decision that screwed over everyone. What all of those points have in common is that the combination of these things irrecoverably ruined the trust in the developer that many players had at that time.
Even with those problems playing MM: DoC can be enjoyable from time to time, but as far as I'm concerned, major updates tend to create more problems than they solve, not to mention that removing what used to be half of the cards available in game from ranked play instead of actually bothering to balance them (and not even managing to tweak the remaining half properly) was probably the most lazy development decision I've ever witnessed.
If you're looking for a more complex alternative to Hearthstone, you might want to check out Infinity Wars instead.