Ok, I can confirm the Neotv 550 has some serious bugs with PGS rendering (playing Blu-ray ISO or folder).
From what I can tell, testing using my HTPC using Arcsoft TMT5 as a comparison, the NeoTV does not show the subtitles on the screen long enough, and this can be a problem for certain titles that show subtitles quickly - for example, several words shown right after another in rapid succession. The NeoTV will blank the subtitle line out for a few frames instead of "flowing" into the next subtitle - so the result is that a short 2 word subtitle that is supposed to be on the screen for 500 milliseconds ends up being chopped into 300 milliseconds.
This also appears to make subtitles show up later than usual at times, and on time at others (Hugo Blu-ray for example - the subtitle shows up after one or two words have already been spoken). Luckily, Hugo doesn't have a lot of dialog compared to the Taiwanese Blu-ray of "You Are the Apple of My Eye", which has rapid dialog with very short subtitles - you will miss some of them and have to rewind to figure out what the characters are saying!
Reference for You Are the Apple of My eye: Chapter 8, scene at the wedding banquet - time: 01:36:11. There should barely be a gap (maybe a couple of frames) before the dialog "That's right", and you just have enough time to read it. On the NeoTV, however, the previous line disappears, and there is a delay of a quarter second before it shows up, and you may not catch that line.
Damn it - I should've bought a Dune - this really makes the NeoTV unsuitable for quite a few "foreign" blu-ray watching - well, at least at 24hz.