Eh, Sens fans have said for years that DJ Smith did him dirty. This seems as much a development failure as anything else TBH.
I'll tell you the 100% objective truth about Brannstrom. It was primarily DJ Smith's fault, not only did he ruin Brannstrom but he probably ruined our whole core.
When he was originally traded for Mark Stone, he was considered to be a top 3 NHL affiliated prospect at the time. In fact it was reported that MTL wanted Brannstrom when trading Pacioretty to VGK but VGK refused and would "only" give up Suzuki.
Brannstrom had one of the best U20 offensive outputs for any Dman in the AHL in 18-19.
After acquired by OTT, the 19-20 season went as planned, got some NHL games (honestly primarily due to injury) and dominated the AHL.
20-21 he was supposed to make the team full-time but got COVID before training camp so he automatically got sent to the AHL. When he came up for 30 games he was 0.5PPG, looked great and was our only plus defensemen besides Zub.
We all thought 21-22 would be a breakout year for Branny but instead he got sent down despite being our 3rd best defensemen. For some reason, DJ Smith would only ever punish Brannstrom for mistakes but Mete, Del Zotto, Josh Brown, Zaitsev could make all the mistakes they wanted with no punishment.
Due to this, Brannstrom started playing a "safe" game and not taking any risks offensively (which was the best part of his game). Instead he was molded into playing as a shut down d who is 5'9 and not overly fast.
He knew that if he made any mistakes, he was benched so played a simple game and you cannot stand out in the NHL that way being an average skater and a 5'9 defensemen.
Despite all of this, Brannstrom did look incredible in some games (in 22-23 and 23-24) and would get the highest TOI with a fully healthy defensive squad. He had the tools to be great but in my opinion was completely mismanaged by DJ SMith.
Just to provide another example, DJ Smith also completely ruined Chabot. Chabot after 18-19 (Guy BOucher was coach) looked like he could be a potential Norris contender in the future could be a top 10 D.
For basically 3 years in a row, DJ Smith played Chabot 30 mins a night in meaningless games. As a result, Chabot would always play at one speed (basically at 75%) which would reduce his overall effectiveness offensively and defensively. He would also make a ton of lackadaisical, high risk, low effort plays, not because he's dumb, it's because he's either too tired or wants to conserve energy. He doesn't know if he's playing 25 mins or 35 mins that game.
To cap this all off, I can even go so far to say that DJ tainted most of the core by never holding them accountable and always trying to be their friend instead of being their coach.