>>> I have a question the other way.  I forgot that `C-x 5 .'
>>> exists and tried `C-x 5 5 M-.' and was surprised to find it
>>> didn't work (it opened in the same frame as if I just did
>>> `M-.'.  Is it supposed to work or should I open a bug?
>>
>> It does the right thing after customizing
>> switch-to-buffer-obey-display-actions to t.
>
> Yes I see that too.
>
>> Maybe its default value should be changed to t?
>
> At least the docstring of other-frame-prefix (and friends) should
> be clearer about it's effect. I don't think I'm supposed to know
> which commands use switch-to-buffer internally.

Maybe more commands should use pop-to-buffer-same-window
instead of switch-to-buffer.  But OTOH, too much commands
already use switch-to-buffer.

> If the default value isn't changed, maybe other-frame-prefix
> should let bind it to t since if you're using it you presumably
> want a new frame.  Same for the window and tab variants.
> project.el seems to do this in project--other-place-command
> which the project-other-{frame,window,tab}-commands use.

I'm not sure if the default value can be changed,
but you are right about other-frame-prefix.
I didn't know that project--other-place-command
let binds it to t.  But doing the same is not possible
in other-frame-prefix, that just sets a display action
for the next command.  So a more elaborate fix is required: