Sunday River is built to keep people moving. The mountain spreads across multiple peaks, leans hard on snowmaking, and gives skiers a lot of ways to build a day. It may not have the remote mystique of Sugarloaf, but it's one of the better Eastern resorts when the goal is getting a lot of skiing done.
The strength is consistency. Early season, lean snow years, busy weekends, and mixed-ability trips are all situations where Sunday River makes sense. You can chase the better pod, shift with the sun, and avoid spending the whole day stuck in one base-area bottleneck.
The tradeoff is that the experience can feel engineered. Sunday River is very good at being a resort, but not always soulful. If you want Maine skiing to feel bigger, colder, and more committed, Sugarloaf or Saddleback may be the better story.