The DL discs are more complicated in structure. There is just a lot more that can go wrong with them, both in burning and in deterioration. The triple layer XL's are just adding gasoline to the bonfire.
I have a fair number of DL DVD discs I burned back, oh six to eight years ago, that are still fine. All of them are Verbatim MKM-001 discs, and I burned most of them at 4x on a variety of drives. The last two cakeboxes I got of these same discs, I started noticing deterioration of data and poor playback very quickly. So I switched from DL DVD's to single layer Blu-ray for larger data storage on optical discs, and I make copies that I keep on hard drives also.
Other people around here with just as much experience as me have stuck with the Verbatim DL DVD's and don't seem to be having that many issues, but I just didn't want the hassle of checking them. I also have the sneaking suspicion that modern DL DVD's are not getting the same quality inspections in the factory that they might have had when DVD's were more popular and more profitable.
But I also noticed deterioration of discs when using Ritek made Blu-ray (single layer). The Ritek BR002 discs have become notorious in our forums for their unreliability. All the data was lost on them in the space of three years for one brand, and in four years for the second (Memorex and Ridata).
Through a lot of reading around here, and personal experience, I've come to the conclusion that there are three well established brands/types of Blu-ray single layer discs that are fairly good. They are Panasonic, FTI/Falcon and Verbatim (HTL only). Melody discs with the INFOME-R30 and INFOME-R40 id codes are also well thought of, though I'd like to see more data over time, so I have avoided these so far.
The only DL Blu-ray with a good rep seem to be the Panasonic discs. And they are prohibitively expensive for my taste.