There are big problems with the transcriptions of a lot of the FindMyPast Welsh records. For example, going for Glamorgan Burials there are 274,705 results. However, there are pages of no names at all. I looked at one and it turned out to be entered in the register as "A Body Unknown". Fair enough, you might say - you can't make up a name. However, if you are a Pay-per-View customer who is looking for a burial you believe may be in a particular parish, you will have to use 20 credits to see the original record or 10 to view the transcription. They say they are supposed to transcribe as seen, but this can be very inconsistent across the record sets - why couldn't they use Unknown Body in the surname field? Some have years of birth entered to match the year of burial for "a body washed up on the shore" with a blank age - why?
The next bad lot are those where there is only a first name. Cardiff St John, Llangynwyd, Abercynon, Newton Nottage, are some of the parishes affected. I dip sampled one from Cardiff and the person's surname was transcribed into the Husband's First Name field. At this point the total changed to 219,603 - worryingly. I carried on dip sampling both male and female first names, and all had the surname transcribed in the Husband's First Name field and therefore blank in the surname field. Page 54 and the total changed again to 223,187. I got up to Charles on page 70 and the total changed again to 236,545 - the surnames were still as the Husband's First Name. Page 76 of who knows how many and it wouldn't show me any more (and now showed 271,959) even if I picked a higher page number, it still showed the page Cornelius to Daniel.
The above records picked were from post 1813 burial registers and were clear to read. It is obvious that there are serious problems with the quality control. The reference at the bottom of the transcriptions says that they are by brightsolid, which is basically FindMyPast.
I then did a search on Abercynon on its own - keeping Glamorgan Burials as the record set. I got 731 results - although this changed to 730 as I went through the pages. You need to go to page 36 of 37 before you get any burials with surnames.
I noticed these anomalies last year and reported them - and I know of several other people who did the same. I also see that it was reported yet again a few days ago on the FindMyPast Facebook page.
You ask how accurate the records are. If you know what you are looking for and when and where, you may get lucky and find it by being careful about not using surnames. If you need to use the surname, you may very well be unlucky. If you are Pay-per-View, I would say be very very very careful or you could end up using all of your credits for very little or no gain.