I'll give a comparison: Valkyries. Why are they so effective or popular? Speed and health. A valkyrie can take a real beating, even by myth units standards. Even better, their speed means you can always micro the valks away from heroes, keeping them alive.
All the myth units people lable as not so good are the ones that die fairly easy. Trolls, wadjets, centaurs, are all much easier to kill with human units than a Valkyrie, mountain giant, or Collosues are. These can all take a good while to take down even with heroes attacking. That survivability means that you are virtually gaurenteed your money's worth. (collosus=most amazing. That lumbering construct is most magnificient when you desperatly need to take down an enemy building. I've had 11 in a long game march under tons of tower and fortress fire and past the opponet's mil to a wonder. The collosus leveled it under the full strength of the opponets mil with 3 collosus left.)
The phoenix, is likely the most vunerable myth unit in the game, despite it being a flying myth unit. If the phoenix was, say, 1/3rd of it's price from the egg or cost no favor only gold, it might have potiental, but as it stands its effectiveness just doesn't justify the cost.
The troll, wadjet, and centuar aren't really that weak. Each of them have specialties to keep them alive, vampire effect(from throwing rocks?! wtf!), regenerating health, and speed, respectivly.More notes: carcinos is probably only second to jormund elver in the water myth units scale, if you don't include 5-headed scyllas. Jormund elvers are really fun and overpowered. Just one jormund elver attacking from the water on medi can do alot, and are jormund elvers, if you can reach them, do quite a good amount of damage to a titan.
Nick: Eten.
Gods: All of them!
Vanilla.