Duma - thanks for the recommendations. ByG is definitely worth a second look when the Wye isn’t feeling like a temperate rainforest.
Simon - possible negative effects: mainly opportunity cost, it’s a distraction from the main (physiological) business of training power and power-endurance. I also wonder - and this is speculation - if training isometrically, at low intensity, with a slow rate-of-contraction, has a small negative effect on “contact strength” ie ability to develop high forces rapidly.