* Needing to constantly monitor whether the fork is being actively maintained, or if it's a vanity project which abruptly stops/slows down updates when its owner/principal contributors lose interest.
* Needing to constantly monitor if the fork is using the latest official Firefox builds to make sure that it's also getting the latest security updates.
* Not being readily able to see a complete humanly understandable (meaning not just comparing git versions) list of changes that the fork makes to the official build.
* Not knowing the reputation of the developers behind the fork.
In sum, I basically trust Mozilla more than I do $random_fork_developer, so I use the official build and carry out my own tweaks, but I am always on the look out for more tweaks, which is why I'd appreciate if lists of privacy tweaks custom builds do were more transparently shared.