It wasn't really a huge loss for the Bruins lol.
Miller was our #3 RHD behind McAvoy and Carlo. He's got some offensive upside, but he plays very sheltered minutes (highest oZone to dZone start ratio on Vegas) and was 2nd on the team in giveaways. 41 points in nice, but the guy was struggling to keep his oiSV% above 90% and he was only a +2 at ES for the season. Good skater, but often skated himself into trouble aka turnovers. Took the 4th most shots on the team last year, but has a 2nd worst thru% (vegas skaters with 40+ games played).
Good player, but he's vastly overrated...especially in Boston circles.
It likely wasn't a huge loss for Boston, considering they have some prospects ready on D, but it's a mismanagement of assets at the least as protecting K. Miller over him is a mistake, even if the plan was to hold on until after the expansion draft and trade him later.
The problem was teams overthought the expansion process by way too much. People thought you needed to be able to protect a player to make it worthwhile to add a player on the cheap prior to the draft, but that was an incorrect way to think. If an addition like TVR cost you a third, then he gets selected, you basically used a 3rd to protect your entire roster. If you already had a player you were going to lose ahead of him, you just got that players replacement for a cheap price by shopping ahead of the draft. This drove the price down ahead of the draft, but Boston could easily have realized some value by shopping later. Even if you think he is overrated, that means someone else would overrate and likely overpay just based on age and position.
Also, some of the stats you use are irrelevant. He was sheltered, but lots of players are sheltered when they are younger. He took on more responsibility and minutes as the year went on in Vegas as was a fixture in their top 4 throughout the playoffs. They gave him lots of Ozone starts, but he was one of their better shooting defenders, so that makes sense, but he was no longer getting primarily minutes against 3rd and 4th liners. He likely is never a shut down specialist, but if he turns into what Torey Krug is, a good 2nd pairing defensive guy, with top pairing offense, he has a lot of value, especially with a right handed shot. Also, oisv% is virtually never attributable to skaters, and is primarily used to see if weak goal metrics are a function of luck more so than skill. The fact he had lower oisv% and plus minus than most of his team typically go hand in hand. They also use it to see if a player was getting the benefit from an unusually high oisv% in their goal metrics. So I wouldn't discredit him based on a lower oisv% for a single season. There is only a handful of defenders in the league who routinely seem to have a significant impact, negatively or positively, on their goalies save percentage, so I wouldn't read too much into that.
This deal is solid for Vegas, as he is getting a little less than typical top 4 money. So if he ends up being a #5 guy with PP upside, who can slide up during injuries, then they are still getting full value. Also, not being locked to the guy into his 30's means they are buying his best years at what will be a bargain deal. So even if some overrate him, Vegas figures to come out OK as this deal doesn't really overrate him, even if he doesn't have much upside from here on out.