Silent Hunter 4 isn't without its bugs and CTD's but the modding community here has helped fix most if not all of the issues. I've dealt with some CTD's in the past and did lose some progress but now that I downloaded some mods it's much better. In my opinion the game is much better with the U-Boat missions as well. I own both SH4 and the U-Boat missions on steam and I have a few mods installed, so far everything works great for me.
If you decided to buy both I suggest using this mod by Webster:
http://www.subsim.com/radioroom/down...o=file&id=1196
It fixes many of the games issues and greatly reduced my CTDs
If you use Windows 7 x64 I also suggest you use this guide:
http://www.subsowespac.org/the-patro...ry-boost.shtml
It will allow the game to access all 4gbs of memory which will also greatly reduce CTDs [Thanks to bifutake for finding this guide]
Since I used the mod and the 4gb guide my game is much more stable, in fact I haven't experienced a CTD since I started using them. Hope that answers all of your questions.