There are some scattered older threads about individual dungeons and a couple confirming some basic elements of how these work, but to make it easier to find I'll add what I learned doing all of them four times to this brief report:

1. All bosses spawn. None are broken/won't appear.
2. Many dungeons in Cyrodiil have more than one boss to beat for getting the completion.
3. If there are four or fewer bosses only one can be active at a time.
4. If there are five bosses (see "Toadstool Cavern") only two can be active at a time.
5. There can be different odds for a boss respawning. Some are more likely, some less, you may have to kill some bosses over and over to get the one you need.

Lipsand Tarn and Pothole Cavern are two of the most frustrating, with Toadstool Cavern in third place.

Lipsand has two bosses and Morbita has a low chance to spawn. You might have to run to the area where is Gaston and kill him several times to get Morbita to spawn. Can sometimes take a very, very long time to complete.

Pothole is similar, with Serrin Vol being most likely to respawn if you are waiting for one of the other two.

Toadstool isn't too bad if you work it right, but it can still take a while. One boss is above and the other four below. Two bosses can be active at any one time, including the one in the upper area. Roreles is harder to get but after killing a few of the others he will show up. This isn't too hard since four bosses are really close to each other. However, one of those four, Serniel, has a *really* low spawn rate.

Other dungeons are more balanced or seem random. Newt Cave, for example, can be really straightforward of you can wait forever for either Rockwing or Gravel Tooth.