IMO the Sun's problem is that they have a roster full of good to very good players 1-12, but no superstar level players capable of taking over a game. In the 20 year history of the franchise (including the Orlando Miracle days) they have had just 2 #1 picks... Tina Charles and Chiney Ogumike four years apart. They had 2 lottery picks in 2016 and drew the two shortest straws, #3 and #4. They have never underperformed long enough to collect multiple elite WNBA talents. By contrast, through years of epic underperformance, the Stars/Aces have had 3 #1s in a row plus the #2 the year before that. Seattle also managed to play under their talent level long enough to get Jewel and Stewie with back to back #1 picks.
As a kicker, historically it has been very difficult to attract the best free agents to spend their summers in rural Uncasville, CT.
This deep but somewhat limited roster results in really good regular season records and lack of success in the post season when opposing superstars take control of the game late and the Sun can't answer. There's no Diana/Brittany, no Nneka/Candace, no Elena, no Stewie, no Angel, etc.
With 5 players entering their 4th year, changes to the roster will have to be made for 2020. It will be impossible to retain everyone we'd like to under the salary cap. By not trading any of these players away early, Miller has essentially gone "all in" with this group for this year's run.
In my opinion, Miller is a tireless worker who is very good at the Xs and Os. He's fine as a coach, although he yells and freaks out a bit too much for my liking. With 20/20 hindsight, deficiencies may have been seen on the GM side at times.