I'm just trying to figure out tradeoffs here. The way I understand sysguard now, it will find out that the load is too high, after the load is too high. You can set the threshold lower to start limiting the traffic early on, but usage spikes will still get you.

I'm trying to compare it to alternative solution which prevents the high load in the first place (puts limit on the CPU/mem shares). By limiting the app rather than nginx itself, you can still set the error page for the case where the backend is too busy (handle 502 bad gateway).

The nice part of doing that at system level is that you can prioritise other processes over nginx, but otherwise ensure the app can take 100% CPU if nothing else is interested in using it.