I haven’t seen any major issues during the latest upgrades (OC 8.2 and later). But that is with the apache-php-module. In the past there have been some reports with fcgi-proxy that were a bit problematic but for me it was not clear if that is due to fcgi or due to “special” configurations of administration tools. So if you want to run several php instances with different permissions, I would take a look at nginx where we have working example configurations.
The snappy-image uses php-fpm as well, perhaps you can get some inspiration from their default configuration: https://github.com/nextcloud/nextcloud-snap/tree/master/src/php/config