There is something rather pathetic and nostalgic about Christie's later books. Nostalgia is not an emotion one can properly apply to events or times before one's birth, but there is a pseudo-nostalgic appeal in these books. At Bertram's Hotel captures this mood perfectly, even if one can quibble about how good the mystery is. The very setting is perfect and logical, even if it is totally unrealistic. Choice of the murder victim is very well done and shows Christie at her best in trying out new plot devices. It is an especially good Marple (even though she has the same age and mind-set as she did in Murder at the Vicarage 35 years earlier, though somewhat less dithery). That business with the absent-minded Canon Pennyfeather is pure delight and a classic invention. This was a hotel I'd give my eye-teeth to stay in, just once.
One thing that always irritated me about Christie was that constant harping about the 'good old days' when servants knew their place, etc. and young people showed 'proper respect' to their elders. Hokum! But as I grow older I can appreciate that attitude more, although not in the same way.
The Pale Horse is a true Christie classic. She was always good with poisons, but this has a new wrinkle on it -- a true 'undetectable' poison (whether that is forensically correct or not is beside the point). The witchcraft stuff is great, and the motivations behind this ingenious business scheme. The book benefits too by the lack of Marple or Poirot, who as detectives had worn very thin over the years. Ariadne Oliver was a good substitute. (I never liked Poirot anyway, except in The Labours of Hercules and a couple of others -- he was rather absurd after The Mysterious Affair at Styles in spite of later efforts to update him.)
If there was a failure in the older Christie, I don't think it was so much a factor of senility or whatever you want to call it than a matter of the 'production-line' syndrome. Christie for Christmas was a watchword, so she'd churn out a book every year, even when 'dry' for that period.