I have spent some time (like an hour or so) looking for a good editor for Chinese script on Mac OS X. To my surprise most of what I have tried sucked in one or more ways.
Of course, I have started with the editor I use to edit plaintext daily - iA Writer. For once, I have said to myself, the humongous font this editor uses would be very useful. Sadly, iA Writer uses the default OS X font for chinese. This derivate of Heiti is quite broken and many glyphs are wrong (this was pointed out to me by a native speaker). Also, line height of lines with chinese characters is weird, and it changes depending of the presence of non-chinese characters in it (including blank space). I have reported the issue to the authors and it was acknowledged so there might be a different font used in future versions… we will see.
So my first choice did not work out. No worries, there are plenty other editors to check. My second choice was MacVim, which is the best invention since sliced bread. The experience was very poor. The insert mode works well, as expected, however the normal mode hates IMKQIM input method. It would seem that Vim was not meant to be used with other than standard input methods, which is understandable.
Moving on. Since alternative software did not work out I tried the native solutions. TextEdit is usually a very good text editor, if you do not need any features. When writing immediately (in plain text mode), you will run into the same problem as with iA Writer : horrible font. You can change the font for plain text mode, of course, but you have to choose the right one. STSong is a good choice, with other fonts you might run into problems with line heights (as some fonts miss some glyphs). Fraise suffers from similar problems.
I have also tried Pages, it works really well, with the right fonts. Line heights are always OK. But it is not a plain text editor, and it has all the downsides of a text processor with hundreds of options.
Finally, I ended up with yet another editor. I have dug up the rusty FocusWriter which I have abandoned previously to iA Writer. The possibility to switch themes makes it a good choice for different scripts. My choice was the 华文细黑 font. Best of all FocusWriter is available on all platforms, so I just use that on Linux as well.
iA Writer no longer has the aforementioned problems, at least in OSX 10.11