By default WP is very reluctant to show or display any errors. That’s inline with its strategy of “dumbing down” things of sorts.
The configuration I use to monitor for errors is along the lines of this in wp-config.php:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_DISPLAY', false );
@ini_set( 'display_errors','Off' );
@ini_set( 'log_errors','On' );
@ini_set( 'error_log','/home/user/logs/php_site_name.log' );
There is also WP native WP_DEBUG_LOG constant, but unfortunately they chose to make it hardcoded to write log into public content directory, which is bad practice.
And yes, overall updating to newer version PHP usually results in breakage and errors if something is incompatible. Not much else to do, short of code review of everything.