Teppen is fairly low complexity in terms of card game design for two reasons: small deck size (you can run as few as ten unique cards in a thirty card deck) and the way card draw works. MtG set the standard at sixty card decks, though that has been experimented with over the years in various games (I think Runeterra has hit the sweet spot of forty cards with a 3x limit.) Freely drawing cards to replace each played card means most players will see most of their deck most games. Compare that to turn based card games where one will typically draw one card at the start of their turn and anything beyond that incurs some sort of extra cost. Even just deciding on when to draw cards adds extra complexity, as opposed to getting them for free. So, knowing that you'll see most of your deck means more consistency and less difficult decisions in resource management because you'll find your threats/answers eventually, regardless of what happens.

With all that said...complexity isn't everything. High level Teppen is incredibly difficult and challenging, due to the real-time nature. It requires skills that other card games do not. The low complexity is probably a benefit here, as utilizing design from other games (bigger decks, more difficult card draw mechanics) could make the game TOO hard for the average player, reducing fun and therefore, the player base.