I agree that a lot of the cheaper ones are cheap because they're run down and need a lot of modernisation - the individual flats I mean, not the development itself.
I've been looking at some and have Rightmove notifications turned on, and probably half the ones coming on the market in over 55s developments probably need £25k or so spending on them to bring them up to a modern standard - some have clearly had no work done to them (no decoration, no new carpets, no new kitchens or bathrooms) for decades, probably since they were first built really. It's usually clear that someone bought them 20/30 years ago, got old in it, and wasn't capable of updating it. Need complete rip out. It's not just the cost, it's the time/stress of getting tradesmen etc, so it's no surprise that the children/beneficiaries think they've made a loss when they don't sell for what they were bought for.
Not particularly McCarthy & Stone, but similar places have the same kind of problems. I've not looked at too many McCarthy & Stone because the place I like locally is relatively modern, so very little "used" ones have been on the market and none have yet become "old and tired" so will still command a decent price when they do, although still a slight reduction on advertised prices when new as McC&S were definitely over-charging for new (but probably lots of deals/discounts behind the scenes like most new build developers).
A few years ago, before I started looking, a neighbour of my MIL bought one and it took him around six months before he moved in - it seemed strange at the time, but he said it needed completely gutting, apparently, mould growing all over the kitchen, food/milk left in the fridge (for years!), etc., "stains" all over the bathroom floor. He was very happy with the price he paid for it, which was something ridiculously cheap, like £50k for a two bed flat - hardly surprising when very few people would want to take on a challenge like that, especially older people! He was "only" 60 so was up for a challenge!
There's another one I've been looking at which has been reduced, not M&S, initially from £100k then down to £80k and currently £70k. I really like it's location, ground floor which is what I want as I want my own external door, but, sheesh, it's awful inside. The block is only around 25 years old and it's hard to understand how it's deteriorated so much - at a guess, I'd say it was only lived in for a couple of years and has been empty for the last 20+ or so, and, yes, mould everywhere in the bathrooms, kitchen, carpets, etc - clearly no one had bothered going in to ventilate it. Definitely a need for full hazard suit when you go in it - estate agents have said as much that they're not encouraging any viewings and hope that people will make offers based on the photos alone!
People really can't expect homes to always appreciate in value, especially when they've been ignored and allowed to deteriorate for long periods of time either because the resident has lost the ability to deal with tradesmen, lost the interest, gone into a care home, or died. Perhaps the relatives/beneficiaries complaining about it should have spent more time ensuring their inheritances were being looked after properly and not being allowed to get into a state of disrepair.