During 90 we measured a lot of spdif output and it was a shame and sometimes a miracle the DAC can lock. Some companies cheat the spdif introducing some components to have a "signature". Other companies like CEC on their expansive cd players used to use bad clock so not all DACs were able to lock. There was a lot of jitter, back clocks and weak signal or .5V instead of full 2V.
https://sites.google.com/site/tvcaudiodocuments/home/spdif-hall-of-fame
Yes, 20-30 years ago external DACs weren't anywhere near as good in locking to SPDIF signals and the signals that did come out of transports were 44.1/16 so DAC's didn't have to do much higher. Some could do 48kHz as well.
The used clock in CDP's wasn't much to write home about but wasn't poor either, perhaps with the occasional exception.
Disc speed ultimately was determined by the masterclock in the DAC part so at least the SPDIF would be fairly stable.
Jitter would be determined by the clock itself and the speed of the clock circuits used which is no problem inside the CDP itself.
Bandwidth limitations, cable imp matching, slew rate of of optical and electronic components as well as the locking capabilities of the external DAC determined how bad the jitter ended up when the internal CDP DAC was not used.
Not much jitter reduction and reclocking was done in external DACs.
An extrenal DAC would synchronize on the received SPDIF clock and often just jitter along or slightly reduce jitter with a slow PLL loop which had its own 'noise' and 'analog' issues.
This made it a hit or miss exercise in those days.
Today, most extrenal DACs have fairly decent jitter reduction, even on optical or electrical inputs. They may not be as good as USB though as different circuits and data formats are being transmitted and decoded. This we can see in many of the tests being done on ASR and elsewhere.
The 'jitter' is less of importance now as it was back in the day.
With a modern DAC and a decent transport (that reads all disc well) you'll be fine and need not to worry. When the transport has issues with discs (ticks, noise or other nasties) buy another one or when it is a really good one have it serviced (when possible).
This leaves me with
@restorer-john 's remarks. He is totally correct of course. There are huge differences between drives in the way they handle tracking, focussing, TOC reading, skip speed, audible noise they make, longevity etc.
This means one can handle crappy discs 'better' than others, some may even play computer made CD's while others refuse to.
Not all CD's are created equal. Those that have actually serviced CDP's (I did) and got sent along problem CD's will have seen the substantial differences in eye patterns and weird 'skips' due to poor adjustment or spindle motor issues.
Certainly when there was a lot of smoking in the household the lenses would be dirty reducing signal.
C2 and CU errors were high leading to ticks or other noises sometimes.
So yes, there are differences between how well CD's are read (remember its a real time thing with no retries as PC CD readout works) and there are differences in SPDIF signal quality. The latter is 'receive side' dependent so stay away from vintage DACs and use a DAC that does well with SPDIF/Toslink jitter reduction and you should be fine.
Is there a difference in sound quality between them ? No... when there is you may have to look for a DAC with better jitter reduction.
The signal path and way incoming data is handled differs between SPDIF (optical/electrical) and USB.