I'm finding mobile web site far superior to app, at least in stability part. I even deleted MU app from my iPad and now 'm reading only comics only on website.
The main problem I have on both is that sometimes, when you tap/click "next" (to go to next chapter in series), sometimes it skips 3-4 chapters.
For example Thor (1998-2004), chapter #10 is followed by chapter #14. I had to google chapters #11-13 and lo and behold, they do exist on marvel's pages, but marvel doesn't think it should offer them in a single series.
Seriously, they should work on the way they sort stuff, it's incredibly messy.