Yes. `--ip` is supported only on user-defined networks. That is because, the subnet for the default bridge (docker0) can be changed (via --bip) or user can change the user-specified default bridge (via -b) on daemon restarts. If the subnet backed by these default bridge's change, then the container with a assigned `--ip` will fail as well.
Having said that, with Docker 1.9 & above, IP address management and Network plumbing are separate concerns and both are pluggable. One could implement a network plugin with any logic and continue to enjoy all the built-in IPAM features (including --ip). Hence, if you have a specific network plumbing requirement, you could easily spin up a plugin (or use one of the many network plugins that are available out there).