I have no experience with the multiplayer side of Construct... except when I caught my students modifying the mp chat example to send messages to each other in class... But I have a couple of questions.
How many players are you thinking of? As a card game, I'm wondering if it's 1v1 or something bigger.
Is player lag even going to affect your players? By this I mean, a lag of a few milliseconds (or even longer) isn't going to be noticeable in a card game like it would be in an action shooter.
Honestly, I think p2p is perfect for a card game. Hell, even a lot of AAA games use p2p rather than dedicated servers. I think you'll be fine.
Of course, someone with more experience on the mp side of Construct might come here and contradict me which is cool.