When you say that you have seen many failed couplings, is it your experience that they fail from ordinary use, ageing out so-to-speak?
SDA: Failed flexible couplings, these vary from an actual coupling that absorbs some misalignment, to insert flexible couplings that are placed between standard shaft and transmission couplings, to Drivesavers. As it happens the bulk of the failures I've seen have been for the Globe Drivesaver, although I've seen failures in all of the above-mentioned flexible couplings. My suspicion is it is misalignment induced, which creates heat and failures. The challenge is the alignment process for virtually all flexible couplings, it's not especially straightforward, and in the case of the Drivesaver, the it needs to be removed to check alignment, so I suspect many of these installations are misaligned.
I have a replacement on the way, two ordered to have a spare on board. Fortunately, on my boat the fix is easy. No more than a one-hour job.
SDA: I would make double sure alignment is correct. Globe Drivesavers are not intended to absorb other than very slight misalignment, their goal is to save the drive in the event of a prop strike (I have never seen one work this way).
Don't couplings have a service life as well?
SDA: Metallic shaft couplings? No, they are forever if properly installed. Flexible couplings are another story, if they are flexing with every revolution, logic dictates they will wear out at some point.
Also, would you recommend removing the couplings and installing a spacer machined to fit that space? I do have line cutters installed so there would be that issue.
SDA: I think you mean Drivesaver, not coupling. Again, I don't see a clear advantage to using the Drivesaver, so you could eliminate it and use the Spurs spacer.