- Inflexible caching
- Hard to move away to another server since it requires a full XenForo install and MySQL write access.
- Constantly changing cache files can be slightly annoying for backups if not properly excluded.
Since the hashing scheme for proxied URLs is well documented, it is easy to replace proxy.php with another image proxy such as Camo+Varnish, however it is impossible to make this new location actually used without core edits.
Such an option would default to: proxy.php?url=$url&hash=$hash, or whatever the syntax is, however other settings such as:
://camo.my.domain/$url/$hash would also be acceptable.