That's actually no longer the case. During EA, there were so many random sudden encounter with no warnings, or very minimal unnoticeable warning. One of the hardest fight was actually an ambush with insane range and the boss having a very powerful source skill.
This has been toned down heavily.
In Act 1 alone
The ambush range has been nerfed. If you proceed with caution, you can even see the enemy first especially if you hold the "Line of Sight" detection.
In one of the key fight of Act 1, they even start talking about how something might join them if they keep fighting.
Both of the ambush warned ahead of time.
...
What two fights are you hinting at? The worst ambushes for me were the frogs early on (you get a warning) and the four skeletons in Withermoore's tomb (you also should be warned by the three-skeleton-fight before). I lost both fights the first time.
I never lost a fight against the Deepdweller on the other hand but I saw him in advance.
At Radeka I knew that bad things would happen but I was surprised anyway. Good fight design, worth an evil witch.
