Nothing is perfect, of course; The Foe From The Future has quite a complex time-travelling plot, with some of the questions raised in early episodes not really answered by the end, and some really rather gory and visceral moments; and like a lot of six-parters from the original show, it could perhaps have been trimmed a bit. The Valley of Death is much more satisfactory plot-wise - indeed, as a story, it is very well constructed - though it would clearly have been preferable as a Fourth Doctor / Sarah story (alas, no longer possible) and has some dodgy stereotyping of South American tribesmen.
Whatever the flaws, both of them are carried by the soaring performances of Tom Baker and Louise Jameson; several of the guest cast comment on just how infectious their energy was for the rest of the team during the recording process, and it shows. Baker is still occasionally silly, but nowhere near as portentous as he has been in the Paul Magrs BBC audios, and also able to effortlessly switch from clown to genius to alien wizard as required. Jameson has finally been given Who material that treats her as an equal rather than as a mere sidekick, and is utterly convincing. And the chemistry between them is clearly several magnitudes better than it was when they were on TV; both stories feature moments when the Doctor thinks Leela is dead or dying, and Baker rises convincingly to the occasion. (The guest cast are all good too, but really it's the stars who I was listening for.)
I have got rather far behind with recent Big Finish releases due to changes of routine and reading and listening projects, but I am kicking myself for not getting to these stories (released in January) a bit earlier; though I think not as much as Tom Baker is kicking himself for not getting into the Big Finish studios when he was first invited. Very glad that he took the plunge and that BF found worthy material for him.