I know this article is from a honda site, but it's actually a pretty reputable site for serious Honda builders. Came across this article, and I was actually surprised to what I read. I assumed NOS would be more dangerous than turbo boost, but according to this, it's not. I'm posting in this forum instead of the 2.0 or 2.5l forums because the explanation references compression ratios, which is usually discussed here.

http://www.hondalife.com/modules.php?name=News&file=article&sid=11

nitrous doesnt raise compression any measurable amount .. it raises cylinder temps. It doesnt "force" air into the motor, just increases the oxygen content.
Nitrous isnt any safer than turbo/supercharging. Its actually a very unpredictable way to add power.

I am no guru and have no personal experience with n2o, but when I thought about it, effective is interchangeable with dynamic. Meaning, the static cr on an engine is just that...when the engine is static, these factors go into an equation and produce what the static cr would be. Using only one factor, forced induction, increasing the load (an increased percentage of oxygen per original volume based on engine speed, which is what forced induction is) you increase effective or dynamic compresson ratio. The main difference is that n2o provides a cooling effect, while turbos and superchargers charge air are heated (hence the need for intercoolers and aftercoolers.)

http://www.resort.com/~banshee/Info/N2O/nitrous.racing.html
http://forums.nasioc.com/forums/showthread.php?t=384611
http://forums.nasioc.com/forums/showthread.php?t=369440
http://forums.nasioc.com/forums/showthread.php?t=400503 (see John Banks comment)

The other issue, often missed in the 'street tuner market', is the concept of Dynamic Compression Ratio. This is your “real” Compression Ratio where you take things like camshaft profile, bore and stroke, rod lengths/ratios, altitude, inlet boost pressures, and exhaust gas backpressures into consideration. The Static Compression Ratio that works on your buddy's EVO motor, or even someone's EJ20 is all but meaningless unless your entire SYSTEM is the same.
http://forums.nasioc.com/forums/showthread.php?t=907570&highlight=dynamic+compression

Nitrous causes much more fatigue on internal components than FI. Your peak cylinder pressures during combustion aren't much higher. All the power comes from an increase in duration of your peak pressure. The metals need time to rebound to their original form between firing cycles and they don't have as much time to do this with N20. This can cause certain parts to still be stressed by the time the next firing cycle comes around.

This is why very stiff (not necessarily strong) components are needed moreso than in FI setups. The OEM EJ257 stuff is very strong mechanically, but not very stiff. If you stripped off the turbo and tried to run 400whp all nitrous on a stock STi block you wouldn't come anywhere close to that. However with a turbo this is a very do-able figure.