I'd love to say "Tittle 18" but in reality and IMO the team lacks chemistry on the offensive end. They lack playmaking, and live at the three point line in leu of working the ball to find the open man with the best opportunity. The offensive is too predictable in that way and easy to prepare for.
A team can live at the three point line if they have Clay, Curry and Durant, but the C's don't. You need to be who you are, not what you want to be. They're a young team and often play like it. In the end shooting percentage will be the main team culprit here.
At the defensive end, youth and solid quick length should pay dividends. Unfortunately in the playoffs the game slows to a grind and interior D becomes valuable. As much as I like Theis, I really wish he were two inches taller and a bit longer.. Plus, he gets the worst whistle in the NBA and needs a solid defensive backup. Naturally Williams will be injured in the first round. Kanter will have nightmares when he sleeps about the pick and roll.
There is a caveat this year, the NBA team that stayed best physically and mentally prepared in this COVID shutdown will have a distinct advantage.
I struggled between second round and ECF loss.
With Kemba, Hayward, Smart, Kanter and Theis, they have a strong vet presence, especially at the end of games. Wanamaker too has many years of pro experience and it shows. I don't think being a young team is going to be their issue.
Also, Tatum and Brown have lead a team to the brink of defeating Lebron's Cavs and going to the NBA Finals already, and have several years of playoff experience themselves.
I think being younger will also physically help them rebound well from the covid-19 layoff going into these playoffs. I would worry more about 35 year old Lebron's body coming off a long layoff going into intense NBA action than I would about guys like Tatum, Brown, and Smart.
You are right about shooting percentage though, that has been an issue with this team for years now. Ainge needs to add some high level shooting to the bench next season.