If you can read the disc, then yes, that should be the case - it depends on the media, how high the errors are at the time you read it, and how good your drive can cope with all the nastiness coming at it. If it cannot recover a block, then no, that data is lost. As per the above, no one knows how long any disc will last - you just play the odds and use good burner's, media, and multiple copies if it is so important to you. Always scan a disc for error's or just a general surface test (CD/DVDSpeed) - it may not be the best, but if you have trouble after a burn, it will just get worse (unless your burner is a bad burner).
One thing that takes time, and is a pain, is to make your own par2's for any kind of data you wish to archive before burning, then add them to the collection (for this to work all files need to be in one folder!!!!) - then, if any file/files cannot be read, you at least have some hope of copying it all to HD and using whatever par2 files are needed to get all the collection back.