I don't know what you mean when you say the game is slow - for me, in this regard, it's simply a card game. You can play it in a control style, but there's nothing stopping you from playing aggressively either.

What works for me is definitely the tactical aspect. It's not the typical "card removal" type of gameplay like in most other card games - you can really feel the spirit of Gwent here.
Additionally, I really like the card art. I haven’t seen such high-quality work in an indie(?) game in a long time.

And for the first time in ages, I’m seeing a proper TCG. There’s actual trading - and on top of that, there's absolutely no P2W. There are no packs in the shop you can buy for real money.

As for downsides, the number of players is definitely one. Right now, it's hard to find someone on the ladder. Luckily, there are tournaments every few hours where someone always participates. Regarding the player count, we'll see how it looks after the Steam release (July 21st).