PHP-FPM is very easy to debug. It's as simple as setting it up to use a listening socket in www.conf (located in i.e. /etc/php/8.1/fpm/pool.d) and then running a packet sniffer (i.e. ngrep) to listen on that socket, all messages back and forth are visible at that point.
I don't know if I'd call usage of a packet sniffer "easy to debug". Seems like it's reasonable to expect some debug option that is easy to activate in dev environments that will give you the relevant information.
The back and forth communication is all that you care about with php-fpm, and rarely do you need to actually see it, unless you have php-fpm configured wrong for instance. The majority of debugging a php application means turning on debugging (i.e. ini_set 'display_errors' On and error_reporting('E_ALL')) and tailing a /var/log/nginx log file looking at what happened in your application.
