I'm fairly new to Pokemon(started playing competitively from gen 6) and here is my opinion on SM.

+Better visuals(although I think the animations were better and cuter in ORAS)
+Game is sometimes challenging
+Some cute characters

-No dex nav
-Very unstable framerate on O3DS
-TOO MANY SHITMONS. Pellipper, Dugtrio, Zubat is in almost every island and you will see them all the time.
-Shitty setup for egg hatching
-No Safari(It sucked but at least safari guaranteed 2 maxed IVS)
-main story is too short
-Cinematics everywhere. It's painful especially at the beginning
-Writing is bad
-Dumb trainer AIs and most of them only has 1~2 mons with them.
-No national dex
-There isn't much to do post game
-Very linear maps with not much to explore
-Hey bro follow the red flag
-Most of the New mon designs are shit
-Most trainer battle starts and ends with you doing a Z move.
-Lillie literally handholding you everywhere
-Rotomdex is annoying
-Taking photos of the mons has terrible framerate and is a shitty system

I think SM is the worst Pokemon I've played so far. It actually makes me think the people who are saying positive stuff about the game is NOA employees.

Here is an image I made to summarize my feeling about the game.