I can’t speak for the PC crowd, but in Xbox world this is one of the best concepts executed in the worst way possible. Being able to join someone’s game is a great idea, but it’s a roll of the dice in more ways than one.
First is the amount of time it takes to load. Holy smokes! We’re talking anywhere from 3 to 5, possibly even up to 10 minutes in extreme cases. The rest of the issues hinge on this one.
Second, as soon as someone begins to join a quickplay, the bot holding the slot they will occupy goes away. So imagine you’re half way through a map, fighting a swarm and/or a monster, then one, maybe two or even three of your bots just goes poof on you, for however long it takes the incoming players to join (see above)!! It’s murder.
When the incoming players do finally join, it’s at the exact location the bot was in when they began to join. Given the time it takes to join (see above), that could be a long way back. Unless the person(s) playing sat on their hands and held tight waiting for you for however long. But I’ve seen people come in on the wrong side of an already used lift with no way up/down to the rest of the party. I’ve seen people join in a half way point of a lift and fall to their death. But most often, they just spawn a mile back and get assasinated trying to catch up.
Another fun one is to join a quickplay, and after waiting however long to join (see above), you’re rewarded with the defeated screen. Because the bot you were replacing was already dead, and the party has since died while you were joining. Best part is, they (or at least the host) were stuck there stewing in their defeat for however long it took you to join, unable to end it and try again.
My suggestion is a lobby, where you form a party of X number of people and then start a map. Then once you start, you’re locked in. Nobody else can join. Then you could even swap heroes in the lobby as needed to form the party, and nobody ever has to wait for that god-awful load in time we currently experience.