I think the truth here is somewhere in the middle. Barnes has been excellent, but the pen after him has been not as deep as last year's pen after Kimbrel, and last year's pen was already fairly suspect.
2018 bullpen (total minus CK): 3.94 FIP, 23.6 K%, 9.5 BB%, .98 HR/9
2019 bullpen (total minus MB): 4.24 FIP, 24.5 K%, 10.4 BB%, 1.21 HR/9
And here's how the Sox bullpen stacks up against AL contenders (and last year's team) without the closer/best reliever from each:
2019 HOU bullpen 2-5 (Pressly, Harris, Rondon, Valdez): 1.81 ERA, 2.64 FIP, 8.54 K%, 2.67 BB%, .32 HR/9
2019 OAK bullpen 2-5 (Trivino, Soria, Hendriks, Wendelken): 3.34 ERA, 2.88 FIP, 9.50 K%, 3.34 BB%, .34 HR/9
2019 CLE bullpen 2-5 (Wittgren, Clippard, Olson, Cimber): 3.07 ERA, 3.03 FIP, 8.15 K%, 2.00 BB%, .67 HR/9
2019 TBR bullpen 2-5 (Pagan, Castillo, Beeks, Chirinos): 2.63 ERA, 3.04 FIP, 9.40 K%, 2.79 BB%, .64 HR/9
2019 NYY bullpen 2-5 (Britton, Kahnle, Holder, Ottavino): 2.59 ERA, 3.14 FIP, 10.82 K%, 3.88 BB%, .74 HR/9
2019 MIN bullpen 2-5 (Harper, Rogers, May, Hildenberger): 3.58 ERA, 3.22 FIP, 9.58 K%, 3.00 BB%, .69 HR/9
2018 BOS bullpen 2-5 (Barnes, Brasier, Kelly, Hembree): 3.71 ERA, 3.39 FIP, 10.95 K%, 3.85 BB%, .86 HR/9
2019 BOS bullpen 2-5 (Walden, Workman, Brasier, Hembree): 2.74 ERA, 3.83 FIP, 10.09 K%, 3.98 BB%, 1.06 HR/9
So, in some sense they're alright, but you can't pretend the signs aren't there. One major issue is that Brasier and Hembree have been too homer prone for their jobs. There's obviously a book on Brasier that wasn't yet written during his dominance last summer and fall. Hembree has always had home run problems, and while his velocity is fine and ERA is decent right now, his secondaries have declined substantially and are getting absolutely crushed this year. He seems to know it. He's barely throwing them.
Workman's had an effective couple months, but that walk rate (6.66, 6th highest among qualified MLB relievers) and low BABIP (.125, 2nd lowest in MLB) require a lot of faith. He also seems to be max efforting every pitch, throwing hard curveballs half the time (third highest rate in the league — Barnes is 2nd). It makes some sense he's going all out because it's his walk year and he's been a near-DFA candidate since forever, but I'm skeptical he can stay on this tightrope into September.
Walden and his slider have been a revelation so far, but he's also a 30-year-old journeyman and has had two bad outings this past week. It's fair to wonder if he can keep it up, or that the league won't adapt to his new slider after a couple months.
Other teams' bullpens have some questionable characters, of course (Cleveland's looks particularly dubious), but most teams are good at keeping the ball in the ballpark. Hembree, Brasier, and to some extent Barnes have been pretty shoddy at that, and the other two guys have virtually no track record of success.