Every so often I go into a normal mission with a perfectly fine group and it just plays wrong. We are running around like crazy, mostly running away from enemies trying to keep everyone revived. Damage isn’t happening as expected. Enemies are way, WAY more aggressive, faster and do more damage more quickly. Most of the time these missions wipe in minutes and they play harder than advanced mode. This happens maybe 1/8, maybe 1/10 missions.
Anyone else?
Yes. It happens once in a blue moon but I chalked it up to that one jack#$$ that runs up ahead where they shouldn’t and mobs start pooring in faster than they would have if the group took their time.
The Sentinel is a good example. First part is easy if you crawl along as a group. But all it takes is one person to run down commando off that ledge and activate all the spawns. Then it’s hell.
In addition to this (i.e. an ally facerushing and spawning more enemies than you would have faced otherwise), some of the packs that you face during a mission are random, which can drastically influence the difficulty of that area.
For example, in Algorithm, in the first boss’s room, many groups wipe to a wave of Ronin, but, sometimes, you’ll get 2-3 Bulwarks (the big shielded cannon bots that can be kited or circle-strafed really easily even if it takes forever to kill them) instead of the Ronin wave. Or a bunch of sniper bots. Or rocket bots (who are only slightly less annoying than Ronin). There’s definitely an RNG element to which enemies you face, which, if you don’t have a good comp for taking on that random spawn, it gets way more difficult.
As an example, I’ve recently been playing Attikus a lot and have no problem utterly destroying Ronin (as Attikus, I’ve single handedly destroyed the entire Ronin wave in a 4 person group and rezzed the 3 others that had all been killed); however, as easy as Ronin are with Attikus, those damned rocket bots are my bane, which is funny when you consider that I’ve basically been able to treat the rocket bots on every other character as little more than the generic strikers.
I just have to ask, how is BB not like Serious Sam? 
Those are difficulty spikes. Typically it’s because you have 4 or 5 people in the group, and it inflates enemy numbers, damage, and health.
Was on Sabo this morning with one other person, in Advanced mode so we were not surprised but…
Three Thrall bosses spawned on the initial starting area and killed both of us.
That’s a very bad location for a fight like that, pinned with ledges everywhere.
(I was Shane, other person was Atticus, two monsters downed.)
(Not Thrall like normally, the Boss variety that do not spawn there.)