The fucking speed

I'm starting with this as this is the primary reason I'm hesitant to play more of that game: it's agonizingly slow compared to older Mario Parties.

I knew the game was slower just from watching videos from the partiers, but being the one to wait on cpu turns is something else.
Everything feels slower, from getting coins from blue spaces, to the abundance of lucky spaces...

Let's be scientific using said partiers. As it's the same players we can assume the skill/decision time is the same between games,
so let's time them from rolling turn order to start of victory screen (to be consistent between separate games):

This video has two 30 turns matches of Jamboree on Roll 'em Raceway and Mega Wiggler's Tree Party (which I'd argue are some of the fastest boards).
We get the rough timestamps 13:48-3:16:25 (duration 3:02:27), and 3:27:07-6:31:53 (duration 3:04:46), for an average of roughly 6 minutes per full turn.
This does contain the Jamboree minigames (which are like 3 minutes each) but I'm currently too tired to count how many happened in each game.
Didn't find a game of them playing on Switch 2 Tag Team Rules (which don't have Jamboree buddies), at least with the same players. Maybe once they get to it on the after party.
Some matches on unmodified Mario Party 6:
- 20 turns on Towering Treetop: 1:32-1:40:22 (duration 1:38:50) for an average of 296 seconds so ~5 minutes per full turn.
- 20 turns on E. Gadd's Garage: 0:46-1:31:32 (duration 1:30:46) for an average of 272 seconds so ~4.5 minutes per full turn.
- 20 turns on Faire Square: 1:12-1:17:02 (duration 1:15:50) for an average of 227 seconds so ~4 minutes per full turn.
- 20 turns on Castaway Bay: 0:49-1:23:05 (duration 1:22:16) for an average of 247 seconds so ~4 minutes per full turn.
This vod is a 99 turns game on E. Gadd's Garage (done with a mod) with a mod to make board movement faster (and also the accumulation of permanent traps).
We get the rough timestamps 10:31-7:45:37 (duration 7:35:06) for an average of ~4.5 minutes per full turn.
I am pretty sure this vod also had multiple breaks but I'm not gonna check right now.

The difference is a lot starker than I expected: an extra 1 to 2 minutes per full turn, which does really add up.

If we are generous and assume the Jamboree minigames are the only source of extra time, given that they are about 3 minutes long (let's say 4 to be generous),
that would be one Jamboree minigame every 2-4 turns, but they're less frequent than that in game (I think it's more like every 6 turns? wiki doesn't have the timings).

Minigames

While having some bad in the minigame pool is a common issue for some mario parties, that won't stop me from rambling, especially cause there are different angles of bad.

Stinky 4 players

A lot of 4p minigames in there are just boring:

Thwomp the Difference feels like waiting an eternity before a reaction check, and for some reason is 5 whole rounds.
I get why the partiers decided to "skip" this one by just all mashing a random button.
Big-Top Quiz same, easy question, lots of waiting, but also you have to move a physical cursor so some players just have an advantage.
Three Throw has exactly the same problem it had on 4: if the players on the edge have the timing, the ones in the center just can't score.
Gate Key-pers is perhaps the worst offender. It's an rng minigame (nothing wrong with that) with an unexciting coat of paint (the most important part of an rng minigame),
and because of "first out wins" and the fact the game still needs rankings to give coins to 2nd and 3rd, 2/3 of that minigame are spent rolling dice to see who gets a whopping 3 or 2 coins.

Duels suck

This game has a whopping 5 duels, 4 if you don't have motion controls, and none of them are good. For reference, MP3 has 10, MP5 and MP6 have 15, MP7 has 12,
MP8 has 27 with some of them being repurposed from other modes, and MPDS has 12 (in board play) which are all repurposed.

No dedicated Battle minigames

The game uses 4 player minigames for Battles instead of dedicated ones. While I'm not against reusing minigames (like Hot Cross Blocks or Sandwiched are perfectly fine),
Battles are high stake and so having them be something as boring as Thwomp the Difference really cuts out the excitement.

The spectacle really matters for high stakes, that's why Bowser's Big Blast is a fun battle but an awful 4 player because it drags on for too long for 10 coins (cough cough Superstars).

Jamboree minigames

Jamboree buddies are the big new mechanic in Jamboree, it's a teammate that can follow you for a few turns.

The buddy doubles up all board interactions, so you get double the effect of the space you land on, you can buy 2 stars at once from a single location, and you can buy 2 items at the shop.
On top of that they each have a unique ability, which ranges from useless (like Waluigi) to very powerful (like Mario or Bowser Jr).

They only last for like 3 turns, but any opponent can steal it from you by just passing you on the board, so high risk of getting stolen but high reward if you can use it.

They spawn periodically on the board, and once someone passes by them it initiates the corresponding Jamboree minigame, with the winner taking the buddy (well initially, they can be stolen).
Here are the issues:

They're around 3 minutes long, which in the context of a minigame is an eternity (you want them more like 30-60 seconds), and are in the middle of a turn so they make it so much longer.
Depending on your board position and roll order, if you win you may just get the buddy stolen immediately before you even get a turn with it.
Whoever initiates the minigame gets an advantage, which on some of them is extremely hard to overcome (at equal skill), thus the outcome is often mostly determined.