Ancestry's recent sluggishness and search engine malfunctions were around before the 1939 register was put on there so I dont think it is to blame for the issues.
I use both Ancestry and FindMyPast side-by-side in my local library and for census records Ancestry has always been my preferred choice until the changes over the last year or so, to the point I've given up using it for anything census related and mainly use FindMyPast instead. The recent free Ancestry weekend had me pulling my hair out.
But after a weekend of looking at Ancestry's version of the 1939 Register I'm pleasantly surprised.
I prioritised finding people who were missing on the FindMyPast version, but known to be alive in 1939. The Ancestry search seems far more forgiving of errors in the data, coming up with results that even though they contained errors, were still better than the FindMyPast ones. The Ancestry transcriptions appear to be far more accurate than the original FindMyPast version.
As a result of my weekend I've now found around 70 families who were 'missing'. The common reasons were mistranscriptions of surnames and also the figure '9' in birth years being transcribed as '0', often (perhaps by coincidence/or because?) the line below was redacted and the tail of the '9' was obscured.
Finding the transcription book cover pages was a plus, but they didn't give details of the parish name, which I'd hoped they might.
I was also surprised the free access ended within a minute or so of midnight.... in the past it seemed like people didn't get round to flipping the switch until the morning.
And it was noticeable that if you are not logged in the Ancestry results page is not much use in comparison to the FindMyPast one. For example it doesn't even give a birth year. I'd need to have a lot more confidence I'd found the right result if I was going to be tempted to renew my subscription on that basis.