It sucks basically.
Here are some things you can do.
Import the subtitles before you put video on the track to prevent timecode mismatch errors
To edit subtitles properly, not one by feaking one.
Export dvd item description for the track you want.
Extract subtitles using "Subtitle Xtractor"
Work with them in "Jubler"
Save as .stl
Import the .stl back into DVDsp
To burn the subtitles back into FCP.
Same as above but import the .stl file you saved out of Jubler in Belle Nuit Subtitler.
Change the look and feel. Check all the text is showing up.
Export titles as tiffs.
Export list as FCP xml.
Import xml in FCP.
Open the xml sequence and copy the titles back on to you sequence.
Copy the titles into your sequence.