The short answer: It's overly simple nostalgia bait with controls that barely work.
The long answer:
The controls are terrible. Well, at least the docked controls are. You're forced to use motion controls that barely work because aiming where you throw your Pokeball is so difficult as to be nearly impossible, so any Pokemon that doesn't stay in the middle of the screen becomes exponentially more difficult to catch. On top of that, the game is really finicky about how hard to swing the controller as anything under the EXACT amount of force means your Pokeball just falls to the ground uselessly and there's an occasional issue where I've had the ball just fly off to one side for no apparent reason. The controls are ill-defined, finicky and awkward.
Battling non-boss trainers is just a complete waste of time. You do get experience from battles, but the amount is a pittance compared to what you get from catching Pokemon. So this means that you're better off walking around trainers that you're not required to fight.
Oh, hey, my Growlithe is a few levels behind. Now I have to empty my party of all Pokemon except for her and start catching Pokemon... which will immediately get added to my party and dilute the experience gains, meaning that I have to go into the menu, pull up the Pokemon box and remove the newly caught Pokemon from the party after every. Single. Encounter.
I sent FIFTY Pokemon to Oak in one go to clear out my Pokemon box of clutter. The people who talk about how you don't have to grind are LYING. The only reliable way to level up your Pokemon is to catch Pokemon. And that means running around an area catching the same Pokemon over and over and over and over and over again until you either get fed up or you're satisfied. By the time I had to catch my 10th Raticate, I knew I was going to have a problem with this.
And since it's what you're doing for pretty much the entire game, I should talk about the catching mechanics. It's Pokemon Go. OK, that's not entirely true. It's Pokemon Go with shitty motion controls! I like Pokemon Go. For a mobile AR game that I can play while sitting down and eating dinner with friends, these mechanics work just fine. For a full single-player console game? Yeah, fuck that noise.
Those last two points together means that you have to rely on doing ONE thing repeatedly to progress and that one thing is pressing the A button/right on the d-pad, swinging your arm awkwardly and hope the RNG gods decide you can have that Ryhorn you're only catching for experience.
And that's not getting into how the Pokemon you catch just feel disposable since you're encouraged to catch them en masse. There's a reason Pokemon Go players jokingly refer to transferring Pokemon as sending them to the candy grinder.
Then there's the fact that it's just a remake of Yellow. And it plays out EXACTLY like Yellow, except that Game Freak seems to have forgotten that your rival is supposed to be the primary antagonist. The story is the exact same story as Gen I, except the driving force of your rival is gone, leaving Team Rocket as the only real antagonists. And the problem there is that it completely eliminates the player's personal interest in the story because, the thing is, YOU ONLY FIGHT TEAM ROCKET INCIDENTALLY.
Oh, but it's actually a sequel because Blue shows up occasionally! And this is frustrating to me because Black and White 2 proved that Game Freak can use the same region for two compelling games. Because if Blue is here, that means that Game Freak expects me to believe that the EXACT SAME EVENTS happened about ten years ago with the EXACT SAME characters who have not aged a fucking DAY. Though, I will admit that it was kinda neat to see Mina from Sun and Moon/Ultra Sun and Ultra Moon after she missed the boat leaving Vermillion city. Granted, now the timeline is even more confused since her design has her as a little girl instead of the teenager/young adult she is in the Alola games, but hey, gotta give praise where I can.
And, yes, I am well aware that story is not a big deal in Pokemon, regardless of Gen V.
What makes this more frustrating that the game looks and sounds GREAT. It's obviously running on the Sun and Moon engine, but seeing this game makes me want an HD remaster of Ultra Sun and Ultra Moon. The full orchestral music is fantastic, too. Plus, having Pikachu hanging on my shoulder as I go is super adorable.
TL;DR: This game is grindy repetitive nostalgia bait made under a false premise that Pokemon is too complex for new players.