The great thing about many Nintendo titles is that the difficulty level can suit casual player and the more hardcore completionist (like myself). One can run through the entire game without too much difficulty, BUT if you want to complete the game in its entirety (mostly collecting all items and finding the secret levels) it can get very challenging. My style of play is to take my sweet time and explore every nook and cranny of every level. I just opened up Dark Side of the Moon (but haven't yet entered, still exploring lol) in Odyssey with over 400 moons. I think from there the difficulty will ramp up quickly.Probably a good thing. I'm assuming you've been around on a few Mario games or so? It may be a bit on the easy side for veteran Mario players, but as I'm sure this was also aimed for kids, it'll keep this from being too frustrating for them. It's probably more important to capture the younger audiences than better maintain the later generations.
Another example is 3D World on the Wii U. I kept thinking to myself the whole time this game is way too easy and seemed very short. Boy was I wrong about the length - in the end there are nearly 100 levels after getting all the hidden levels. This is one of the longest Mario games so you're getting a lot for this purchase. The final hidden world (world-crown) is very difficult. This was excellent as I was playing with my kids in the earlier levels. The later levels they couldn't handle and they jsut got in my way lol. Nintendo really should port this over to the Switch so all those who skipped the Wii U can experience it.
Regrettably the Galaxy series I never got into, mostly because of the forced motion controls (hated having to point the wiimote to collect those star bits). 3D Land on the 3DS I had to do it twice, because I inadvertently used one of those assist blocks and I couldn't get the 3 stars on that save game file. So I had to start a new game just to make sure I end up with a 3-star save game lol.Of all the 3D Mario games, I've only played and completed Galaxy1(Wii) and 3D Land(3DS); so not a whole lot of experience with Mario games.
However, the difficulty of Odyssey does ramp up after the campaign.