My return rate is not particularly alarming -- probably about 1%. (That's in terms of customer orders, not disks. Many customers order more than one disc.) I probably mail DVDs to 100-150 customers a month.
What concerns me is that most problems are from Toshiba or Samsung players. I'm not by any means suggesting that all (or most) Samsung or Toshiba players are a problem or can't play all kinds of DVDRs. I also know that there can be variations in the production run of the same model number, depending on when it was actually produced. The thing is, two manufacturers stand out disproportionately for problems.
For example: Samsung P-421, Samsung DVDV4600, Samsung DVD V-2000, Samsung DVD-V3650, Toshiba SD-3960SU, Toshiba SD 3800, Toshiba SD-2300 owners have had problems. And there was one Maganavox that was manufactured by Samsung.
OTH, I have never had a return from a Sony, Phillips, LG, Pioneer, Denon,Yamaha, or Panasonic owner. One Toshiba "combo" owning customer bought a 24 DVD "library" from me & emailed me that most of the disks wouldn't play properly. I suggested he buy a $75.00 Sony, he did & they all played perfectly. And as I said, I have had several instances where a customer who owned one of the machines I listed above had problems with my +R discs that had the booktype changed to -ROM, but had no problem with the DVD-R replacements I sent them.
I have 4 players -- a fairly new Sony, a new Philips, a Pioneer & a 6 year old Panasonic. Almost all returns I get will play on the Sony, Philips & Pioneer. A few more have skipped on the 6 year old Panasonic
As for as Verbatim vs Ricoh. Using Ricohs, I had no customer complaints for three months. When I switched to Verbatim, I got 3 in three weeks. I know that is not a huge sample or a high return rate, but it it's enough to tell me that the Verbatims are giving me more trouble, yet the Kprobe results are still very good.
I offer "no questions asked" free replacements, and have always been able to come up with a solution, but for every customer who complains, there are probably just as many who don't bother to say a word and never come back. That's why this problem is so irritating.