I suspect the problem with them is that some people have no issue with them at all, but for people who do have a problem the problem is that they are extraordinarily uncomfortable, and can result in pain which can last for hours afterwards. I think for me it's probably the way that it actively forces my back into a position my back won't normally go. It's entirely in how upright the back is.I 100% genuinely I don't get the hate with IET seats. Shows how subjective it all is.
Is it down to people wanting to slouch on the seats or something?
I've done Aberdeen-Kings Cross followed by Paddington-Penzance before without the slightest hint of discomfort beyond the sleep deprecation of having roughed it without a bed on the ferry to and from Shetland for the previous two nights. I do lengthy journeys on the TPE units with the same seats too, and also have no issue there. Also, they don't seem to attract nearly as much criticism on TPE (or Hull Trains for that matter) as they did on GWR/LNER for some reason.
I've seen posters on here say they're fine for some people but bigger built people will hate them. I'm not exactly light and have no issues.
Fainsas are very firmly-padded and not-exactly luxurious, but that's also somewhat true of most of the Leans, and to an extent also the refurbished seats on mark 4s. I can travel 5 hours on poor padding, but I just can't travel on LNER standard class anymore.
All beside the point because nobody is threatening to put Fainsas into XC stock.