Varnish Cache is quite easy to install and make it run, but is it easy to debug as well? The answer is “meh”. It can go either way.
Yes - because the Varnish tools you have at your disposal (in particular Varnishlog and Varnishstat) will give you a lot of information regarding your Varnish instance and with some patience you’ll be able to understand what is going on.
No - because if the strange behavior/crash is triggered by a peculiar scenario, then it will be difficult to reproduce, and likewise difficult to understand and fix.