I think surprise traps can work within certain bounds. Let's take The Machine as an example, a mostly good one. There are essentially two surprise traps in the level: ghost spike bars and retractable spikes.
First of all, Knytt Stories in general doesn't have lives or a score, thus, provided that save points are properly placed, getting killed by a surprise trap isn't usually a big deal.
Ghost spike bars are always surprise traps, and they're the 'worst' example of that in The Machine, though that's not saying much: they can't be detected by the Radar, but they're confined to a single area, so the player knows that s/he has to stay alert only in that area (it's still a pretty large one, so it can get annoying). There's also no instance where you can get surprise-spiked after a challenging section.
Retractable spikes are used sparingly, only appear on certain types of platforms (you can always tell where they'll pop out after the first time), and the first time you see them, you probably won't get killed by them (so it's not really a surprise trap, strictly speaking). The Radar works on them, but that's not necessarily a factor, since it's an optional power-up.
Finally, all in all, there's a limited number of surprise traps in the whole level, and a limited variety: after the first death, any player would know what to expect, and where s/he can let her/his guard down. There's also no 'warning' Save Point before them, if I remember correctly, because there's no need, as the nearest one is a couple of screens before the trap, with no hard challenges in-between (no frustration, yet no 'surprise spoiler').