The sleep quality of a nap depends on both practice and placement.
For example, in the Uberman schedule, not all naps are REM, it depends on the placement what type of sleep an Uberman will get in a nap. One might get 3-4 REM dominant naps and 1-2 SWS dominant naps, and of all 6 he might get 1-2 mixed quality naps.
In the DC1 and E3 schedules, you may find the naps are all NREM at first because napping has not yet been practised (and there has been no sleep repartitioning). When you practice (after a few days or weeks of napping) your sleep pressure will build and your sleep stages will repartition so that any naps that fall within the circadian REM bracket (pre-dawn to midday) become REM-dominant naps. After this change has occured, if a nap is not REM, then placement matters.
The circadian rhythm will dictate REM quality sleep from around pre-dawn to midday, and will dictate NREM quality sleep from around afternoon to midnight… this assumes a practised sleeper.
Tying in the understanding of BRACs and nap placement
Your BRAC cycle over the day will dictate the quality of sleep you are getting, and you may be able to detect undulations in your concentration or wakefulness throughout your mornings, and likely these dips in concentration are perfect times for a nap (they will yield a high percentage of REM or SWS).
If you have set nap times, and those nap times are at the peak of a BRAC (high concentration or high wakefulness) then you will likely get light sleep if you fall asleep. There is no guarantee that an equidistant napping schedule such as the Uberman schedule will yield optimal nap placement. You may find you get a better yield of REM or SWS and less light sleep by breaking away from your ultradian rhythm and placing two naps close together, or two naps far apart, stacking naps when sleep pressure is highest.
For example instead of 6 naps placed equally over the day, you might find optimal placement looks something like this:
You can apply this idea to any schedule, really. If you find you are consistently feeling tired at a certain time before or after your nap, or a nap is unrefreshing (or a sleep recorder like Zeo records light sleep) then it may be worth changing a nap to a more optimal time of day.
Try pushing a nap forward or backward half an hour or an hour so that you maximize the amount of REM and SWS in your schedule.
Extending and Cutting Sleeps
In some circumstances extending a nap or sleep can be worth it. If you can get 20 minutes of REM in a nap, then you should experiment with extending that nap by 10 to 20 minutes more to see if your sleep changes back to light sleep after this point, or if it continues to be REM sleep. If you can get 30 or 40 minutes of REM in a well timed nap, then you may be able to cut back another nap that is showing less sleep quality, or you may just like to enjoy the health benefits of the extra REM or SWS you have gained by optimally extending a sleep.
On the other side, you may find cutting a nap, or splitting a core sleep yields a higher percentage of SWS or REM sleep. Consistently getting 30-40 minutes SWS at the start of a core followed by broken SWS/REM/light sleep may mean you should only need to sleep for 40-50 minutes at that time. You would split the core sleep up into two core sleeps to see if you can get two solid core sleeps rather than one poor quality one.
Increasing sleep quality
If neither extending nor splitting your sleep is increasing your sleep quality, then you should look to other problem areas such as your overall sleep hygiene, or other medical problems. You may find that you are only getting small amounts of SWS at a time (followed by broken SWS or REM) because your blood sugar is unstable, your lighting conditions are not right, you have a nutritional deficiency/toxicity or some other problem that is effecting your sleep quality.
Fixing a problem that is unrelated to schedule formation may change that small amount of broken SWS or REM into a long bountiful sleep.