There is one thing about The Emerald Tiger, by Barnaby Edwards, which is quite remarkable and superlative: the incidental music, by Howard Carter, is some of the best I can remember from any Who story ever. I see that Carter has done music and sound design for several other Big Finish stories, none of which I remember grabbing me as much; perhaps they just gave him his head for a change. Here's 40 seconds of a particularly thrilling passage.
As you can tell from the music, the setting is India, in 1926, and the Doctor and colleagues end up in an exciting chase by train, car and balloon of dubious Brits and a large telepathic tiger to a lost valley where mysterious things are afoot due to a local infusion of handwavium (not what it's actually called but you get the idea). All jolly good fun, with the trememndous energy of the music reflected in the performance of regulars and guest cast (who include Cherie Lunghi).
I'm afraid that The Jupiter Conjunction, by Eddie Robson, didn't work for me as well. I was simply not convinced by the astronomy and physics of the setting, a human colony on a comet which has been used to ferry supplies between earth of Jupiter and now faces crisis because the planets will not be aligned correctly in future. It seems to me utterly implausible that one might use a comet for this purpose - no reference is made to the rapid rotation, the low gravity and the icy, crumbly, volatile surface; and even if anyone ever did, the positions of the planets are not exactly unpredictable and would be built into the business model from the start. If you can swallow the background (and most listeners probably will) it's a decent enough tale of aliens being exploited by human allies and vice versa, with comedian Rebecca Front playing the chief villain, and some excellent sound work on the alien voices. Most people will like it more than I did.
But then we are back in business with Marc Platt's The Butcher of Brisbane, a prequel to the classic Fourth Doctor story The Talons of Weng-Chiang, telling the future history of how Magnus Greel (played by Angus Wright) got his alien sidekick (played chillingly by Rupert Frazer) to design the Peking Homunculus, and eventually fled the ruins of the 51st century leaving conflict and destruction behind him. The hints that we are given in Talons are considerably augmented by Platt's vivid imagination, in particular how he manages to insert Nyssa and Turlough into Greel's inner circle; the world of Magnus Greel is credibly sketched out. It would have been very easy to write a bad prequel to Talons, and we have instead a good one - certainly the best of these three.
(Though the story's treatment of the Time Agents seemed to me at odds with what we know of them from New Who and Torchwood.)
So, in summary, The Butcher of Brisbane is the best dramatically; The Emerald Tiger has superb music; and The Jupiter Conjunction is not recommended for astronomers. I think all three are fairly penetrable for non-Who fans - the Talons of Weng-Chiang references in The Butcher of Brisbane are important only for the choreography of the dénouement.