Honestly, the newest games can feel overwhelming due to the sheer number of additional features, items, and pokemon that are now included.
Features-wise, just off the top of my head there's Poke Finder, Festival Plaza, Poke Ride abilities, Pokemon Refresh/Poke Amie, Poke Pelago, trainer customization, shinies, passport pictures customization, ribbons, berry harvesting, hyper training, EVs/IVs/natures, the QR feature, Zygarde cells, Totem Stickers, Ultra Beasts/Ultra Space/Ultra Wormholes, fishing, the Battle Tree, the Battle Agency, Furfrou haircuts, Mantine Surfing, Z crystals which open the door to a plethora of new moves, Mega Evolutions, a lot of daily events, the complicated mechanics involved in breeding, all the online trading and battling options, and a plethora of other things.
Itemswise, since there are now 802 pokemon, in addition to all your story-, healing, ball, and battle-related items, you have all the extra items that go with specific pokemon: things like the drives to change Genesect's type, the memories to change Silvally's type, all the form-changing items like Shaymin's Gracidea, Marowak's Thick Club, every single Mega Stone which uniquely corresponds to one Pokemon. And people give you these items constantly throughout the journey, so by the time you reach the Elite 4, your bag is a bloated mess that takes over a minute for to scroll down and find whichever item you're looking for.
And then there's the pokemon themselves. We're at 807 now, not including all the additional forms and Mega Evolutions. If I recall correctly, there were 395 pokemon available in the (non-Ultra) Sun and Moon. That's an insane amount to hunt and find (taking note of rare pokemon and the myriad of unique evolution methods), and that's not even half of all the pokemon there are! But if you want to get into competitive battling, the sheer number of options makes for a lot of pokemon, movesets, stats, and abilities to get down (granted that only a fraction of all pokemon are relevant in any given tier).
Now, I'm not really faulting Game Freak for providing so many different ways to play and enjoy Pokemon - in fact, that's what I think is a huge strength of the series that contributes to its longevity. But there's definitely a "design through accretion" philosophy in place which can make the newest games daunting for someone just starting to get into the series. And, despite some beginner-friendly safeties like the EXP share, with so many options for pokemon teambuilding available and so much knowledge required, it can be easy for a new person to make choices that make even "beating the game"/becoming the champion difficult. Nevertheless, I would encourage a new player to figure out what they like, and enjoy the wide breath of experiences the pokemon world has to offer at their own pace. :)