The first thing I would do is update nero if you have not already. Earlier versions of nero 6 had a lot of issues with encoding diffrent file types. I think they have the majority of the problems fixed in nero 6.6. Unfortunatlly, my experience has been the same as yours when trying to convert many diffrent file types in one shot. I have yet to find an all in one shot program like nero that is up to the task. Your only choice may be to convert files individually and then assemble them into dvd format. It will give good results but is not quick and easy like nero (it was too much trouble for me so I didn’t bother).
Here are a couple of things that might help though. First, update nero like I said. Second, if you have a lot of files and are having problems with a group of files, don’t sit thier continuing to try those files. Move on to other files. I fought with nero and wasted a lot of time trying to do this, just to realize that when I moved on to other files, thier were actually just a few files that gave nero problems. I was able to burn a lot of the stuff I had with no problems. Getting a lot of your stuff burned onto dvd is much beter than getting none of it done. Also if it doesn’t matter what order they go in or whats on what disk, try to group similar file types together for a particular conversion session. Nero seems to do much beter when there are not too many file types at one time. Maybe if you get lucky like I did, you can get most of your stuff burned with nero. As for the rest (files that nero has problems with), you can try one of the other all in one programs that work similar to nero. Not that they are beter than nero, but sometimes nero will like files they dont and sometimes they will like files nero doesn’t. If several fail, you might have to do some the hard way. Keep in mind, if these are avi files from the internet, p2p or torrents, it is possible that thier is some coruption to the file that is causing problems even if it seems to play fine. If 9 out of 10 files are good, it might be worth trying to get another copy of that 10th file. Unfortunatlly, if nero hits a bad file, it will usally stop so you don’t know if the rest are good. About all I know to do is try again and omit the file it got stuck on, or move on to the next group of files.
I know that didn’t give you a solution to your problem, but hopfully it will help a little.
Fyi if multiple programs are having problems with a group of files, it is liklly that thier is some file coruption (or maybe missing codec’s but usally you will get a specific error about unsuported file format if it is a codec problem).