The only case I can think of, which creates problems when "justing ticking APPI/URLF blades on" is:
You have an Access Policy, which has an ordered Application layer in it. I saw this in environments, that were upgraded from R77.x to R80.x. somewhere in the past. APPI/URLF was never used there, but the conversion process by upgrading Firewall Management to R80 back in the days, created this legacy Application Control ordered layer in that Access Policy. It was not doing anything as long as APPI/URLF blades were not installed. After installing them, APPI started dropping traffic based on the one and only (I guess default) rule in that Application Layer.
It cannot be an inline layer with that feature or unified access policy, because SmartConsole would not let you upload that to a gateway which does not have these blades installed.
So just verify that your access Policy does not have an (legacy) Application layer and than you should be fine in activating these blades.