In theory this is based on any mini game but I will use Dragon Ball as example because it made the problems more obvious.
I waited until now to make this thread since the event is over and I got all the available festival achievements so I hope to lower the amount of posts of "lolol you just can't play"
1. They are using the Hotjoin pvp mechanics: The Red Team will always get the priority when new players join which usually means Red gets an edge over Blue.
A typical scenario is a match that starts 3 vs 2 - obviously the 2 Blue players will get slaughtered. Red will quickly lead in points and any 3rd player who joins will leave again because he doesn't see any point in investing time in a match which was lost before he even joined.
2. HoT changed the instance mechanics so instances will always try to fill up.
For a mini game like Dragon Ball that means the instance won't close but the game will keep trying to add people to the Blue Team.
Those added to Blue will leave as said so the game will use another Hotjoin feature and "offer" the Red Team to volunteer and change sides for a promised free win. Too bad that this doesn't work - if you volunteer you will not actually get the win so one player of the winning team will get busted.
So there are only very few Blue players left if any and the Red team has to keep going and try to score 500 points to end the game.
No opposing players = no points to earn
They can only hope that a few Blue players will stay and volunteer to get killed to end the running match.
There is no "surrender" button or any mercy feature.
There is no feature that keeps the additional player inside the base by tossing him back like they do when someone leaves the base before the game has officially started.
TL;DR: The mini games would just need a slight touch of customization for balance and they would be great but Anet never bothered to do that and seems to have made it worse recently.
Harbinger of Fools