Awful. The roster is, and has been, the problem.
I mean just look at the C's roster the last time it went to the ECF just 2 seasons ago (excluding Tatum and Brown).
Hayward, Walker, Theis, Smart, Freedom, Semi, Grant, Robert, Wanamaker, Romeo
And that was a fairly big drop off from the 2 seasons before that when Boston had
Irving, Hayward, Morris, Horford, Smart, Rozier, Baynes, Theis, Semi, Wanamaker, Robert
The simple reality is the roster has been hemorrhaging talent for years and is just awful.
Boston is quite simply a bad team talent wise past the top 2. I mean look at the Cavs. Taking aside Sexton after Allen and Garland they have Mobley, Markkanen, Love, Okoro, Osman, Rondo/Rubio, Wade, Windler. And no one trusts the Cavs, but that is their roster taking away their leading scorer of the last couple of seasons who got injured this year.
The Bulls outside of DeRozan and LaVine have Ball and Vucevic plus Caruso, P. Williams, White, etc.
The Bucks have Holiday, Lopez, Portis, Allen, Connaughton, Hill, Matthews, Semi, DiVincenzo (injured), Hood, etc.
Even the worst team in the league i.e. Orlando arguably is better 3-15 than Boston so if Anthony and F. Wagner are 1 and 2 that gives them Carter Jr., Harris, Bamba, Suggs, Ross, Okeke, Hampton, M. Wagner (I think Boston is better, but not so much better you couldn't have a discussion especially since there is no one like Jalen Suggs on Boston's roster).
Boston has a bad roster which is compounded by being led by two all star caliber wings that are both best suited at SF and who occupy the same space on the floor (i.e. making them ill fitting as a duo).