Up: It's a standalone series in a shared universe
Frank Castle is no superhero. As a mere human whose special ability is a high tolerance for pain, he can't hold a candle to the likes of Luke Cage. The show's producers were aware of that, so Castle lacks the presence of the other Defenders.
But that works well in this first season, because it makes the story more intimate. The inclusion of Karen Page is the only hint that The Punisher exists within a larger shared universe. Bernthal's scenes with Deborah Ann Woll don't feel as if they were worked into the show just to demonstrate a connection with that universe. The scenes work well on their own, even for viewers who may not have seen other Marvel/Netflix shows.
For the most part, the season stays on track. It becomes clear that The Punisher isn't a superhero. He's not even a hero, but he's not a villain either, although he's done some horrendous things.
Even better, unlike most of the other Netflix series, there are no ninjas in The Punisher! There's no hint of the The Hand, which was getting boring by the time The Defenders hit. No, this season is sharply focused on learning who murdered Castle's family.