When NextCloudPi first came out I was bringing the packages from Stretch into Jessie in order to use PHP 7.0, instead of the PHP 5.0 that ships with Jessie.
That worked mostly fine, but there were some weird incompatibilities from mixing packages ( php-smbclient, for instance ).
That being said, if the benefits were really important it could be done again.
well noob that i am i don’t know about any major changes that could turn out to be important. unless this one app - “password” - isn’t working with php below 7.1