Mario Party 9 and 10 are underrated games. I've seen gameplay of them and they seem great!
As someone who's played all but two of the Mario Party games and who spent months on a video script ranking every minigame from 14 of those games, I'll defend them from the "Mario Party 9 and 10 are some of the worst games ever made" crowd all day but I can provide a somewhat big list of reasons why they're not good as Mario Party games.
MP9:
-The car is awful. Not only does it remove any semblance of strategy because you have no control over where you are at the start of your turn, but it incentivizes speeding to the end because bigger rolls means a better chance of something that will put you in the lead.
-A side effect of the car is that party mode games end in like 15 minutes sometimes, maybe 30 minutes if you're playing with other people. That's pretty pathetic when I could boot up any game that came before it and play for multiple hours. I have the option to do 10-turners if I want but that's not literally every single game.
-ND Cube is really obsessed with side modes. This is a problem that's never really gone away, but at least in those cases they actually had the time to develop the board play before they went to the side modes. For 9 though, if the only reason I ever feel like going back to it is to play Garden Battle, what's the point of calling it a Mario Party game?
-This game was released during peak Toadification of Mario games and it shows because this was the first game to introduce Toads as hosts alongside Toad as a playable character. This has never changed, with the exception of adding Kamek as a side host, and people continue to not like it for a reason.
-9 has a reputation of being the first "bad" Mario Party and for ruining the series. I would disagree with that because I'm not a 90s kid and I don't think the series peaked with 1 and 2 but it is a reason it gets so much hate.
MP10:
-No one liked the car from 9 and yet
that's what they chose to bring back of all things, which also comes with all the problems it had in 9.
-Mario Party 10 reeks of being rushed. It has less content than either 9 or Island Tour (which came out on a handheld so there's no excuse for that) in both boards and minigames, and Bowser Party is only playable on three of the five boards.
-Bowser Party is a cool idea but it's really just a glorified side mode. Its execution was also pretty bad since it's so stacked in favor of Bowser. Like, if you get any roll that isn't enough to catch you up to the car as Bowser, you can just do it again and I think that's stupid.
-To add to the rushed vibe, this was the first mainline Mario Party, if not the first Mario Party at all, to not have any kind of story mode. If you're a single player like me, it can really help to have a mode designed for single players but that just didn't exist in MP10.
-All the board themes feel so uninspired and NSMBified. Gone are the days of fun places like Faire Square and Neon Heights; now we have standard spooky forest board, standard underwater board, and standard Bowser board.
-amiibo Party is just...bad. It's already physically paywalled behind amiibo but when you can open it, you're greeted with an extremely uninspired bastardization of traditional Mario Party board play where everyone is either amiibo figures or cardboard standees and you're just playing on a square. It also makes you scan your amiibo far too often for it to be fun as its own thing.
-Mario in the mid 2010s struggled with a lack of personality and dear god, MP10 is no less guilty of that. I may hate Mario Party 1 and think it's aged extremely badly but at least you got some fun menus to make your experience that little bit better. Even in Mario Party 9, you could watch the characters wandering around in the background. But in 10, you open the main menu and you're greeted with static rectangles and a bland background with nothing going on.
-There is one (1) song that I find memorable from the MP10 soundtrack and I think it's used in like, two minigames. Even the instructions screen music is just dull compared to what came before it.
-This is a lot pettier but why did they try to make Spike a thing? I think it's funny when they try to make the most random characters a thing (looking at you, Pom Pom in Super Mario Party) but it's just a strange choice. And even crazier, it kind of worked?
All that being said, I really think ND Cube knows how to make a good minigame. They have their fair share of stinkers but I think 9 and 10 genuinely have some of the series' best minigames. 9 has Tumble Temple and Goomba Bowling, and 10 has Flash Forward and Soar to Score. 9 also has a pretty decent soundtrack and I really do think they wasted the Bowser Jr. boss music on that really anticlimactic final midboss minigame. Other highlights include (and apologies that I can't remember their names right now) the one from the second half of all the other midbosses, Speeding Bullets, and the minigame instructions music. The problem isn't that they're bad games in a vacuum, it's that they're bad Mario Party games.
E: And if you don't believe me that the car was a bad mechanic, I can prove it was by the fact that they've released two original, much better received Mario Party games without the car. Super Mario Party had pretty mixed reception but one of the most common opinions I saw was that it was a step in the right direction to get rid of the car and move back toward traditional Mario Party. Then Jamboree came out last year and pretty quickly became like, my third favorite game in the series.