The only two brands that have NEVER given me a playback issue have been Taiyo Yuden and Fuji dash-R. I have had luck with Memorex, but have also purchased cakeboxes with apparently bad runs.
Like you I have a couple of pro players, and three consumer decks - even a cheapo $27 dollar player from WalMart that has played everything I have thrown at it. So, after testing my disks on several, and playing them through on the cheapo, I would assume it is the fault of the client's player, not the disk, and suggest they run a cleaner through the system, or ask them how old the system is and if it is more than a few years, suggest they might want to test the DVD on a friend's or another family member's player, or even take it to an electronics store and test it. I tell them that because it plays on everthing I have, from expensive to cheap, and my computer, and works in my duplicator, I HAVE to assume the fault lies with their player in some way or another. Clients might not LIKE that suggestion but often that IS where the problem lies.
In one instance several years ago I brought three of my players to a particularly troublesome client's home, hooked them up, even brought a portable one - all played, still theirs didn't. I asked for permission to run a Philips brand DVD cleaner through theirs and voila, it played. Everyone was happy.
Occasionally, also in years past, when delivering projects in the $1.5K or better arena, I've included one of the WalMart $27 players with the package. I've shied away from doing that for the last 16 months or so.
At one time I used plus+R disks exclusively because I was using Philips brand burners that called for them, but I have long since converted to dash-R exclusively. I also, however, never had a client playback issue with the TY or Fuji plus Rs either. I have tried virtually all other brands from crapocheapo to Watershed TY and gold standards, and ONLY the TY and Fuji have been totally trouble free for me.
I don't know enough technologically about the difference in the two formats, their burning processes or laser elements to respond to your last paragraph. Sorry.