I feel like I've done this before but I can't remember where so here it goes again:
Rick Adelman
Adelman coached for 21 years and missed the postseason a full third of his career, 7 seasons. He only made it to two Finals, with those great Blazers teams. Never winning a championship could be forgiven if he made it to more Finals or conference Finals, he only made it to four and was 2-2. Adelman had two 60-win seasons and nine 50-win seasons with six sub-.500 seasons...
In 14 playoffs, he was a Rd1 out 8 times, over half his playoff career. What does that say about Adelman?
He has a .582 win percentage in the regular season, which is equivalent to 47.7 wins/season. That isn't bad but that's not even 7 games over .500, he coached his entire career in The West, in an average year that's around an 8-seed in The West. 40% of the time during his career, you're missing the playoffs out west with that record (~47-48 wins), and you're never a team with home court with that record...
So a guy who on average was a low playoff seed, missed the playoffs a full third of his career, was one and done in the playoffs more than half the time he made it, never won a title, was .500 in his playoff career, only played in two Finals, was .500 in just four conference finals, this is a Hall Of Fame coach?
For comparison, Doc Rivers has coached 22 years, so roughly equivalent to Adelman's 21. He's only missed the playoffs 5 times with one more season coaching, been to the same amount of Finals but won a title so he has an edge there; has a winning conference finals record (2-1); and has also been a Rd1 out 8 times. Doc has two 60-win seasons and eight 50-win seasons with three sub-.500 seasons...
Doc's regular season record is .585, 47.97 wins/year, so same as Adelman, with a .506 playoff record, also same as Adelman's .503. His record is on average a 5-seed in The East, where he's coached 14 of his 22 years, and on a good year that record is good for a 3-seed out east...
So is Doc a Hall Of Fame coach? He's had the same career as Adelman while spending majority of it in the easier bracket. What distinguishes Doc is the championship, losing his other Finals down to the final minute in a G7, and winning a championship with All-Time superstars while Adelman couldn't seal the deal with his superstars. Doc also had half the sub-.500 seasons as Adelman, so I think he has enough of an edge to be a legitimate Hall coach...
Adelman was a good coach. Too many red flags to be a Hall Of Fame coach, though...