This is a really interesting thread!
Having taught for years, I think problems arise as soon as something that is, at its core, quite a good idea gets given a snappy title ('grit and resilience') and gets turned into a sort of problem-solving product. The purpose gets twisted and becomes something to serve the organisation and not the individual.
Mindfulness is a good example. I've read enough about it to know that, done right, mindfulness techniques are helpful for many people. However, every time I've encountered it at work, it's being suggested as a low-cost sticking plaster to give the appearance of 'doing something' about low staff morale/mental health. When actually, the reasons for the low morale are terrible working conditions, long hours, inadequate pay etc. It stops being about helping the person and more about making that person the problem so that the organisation doesn't have to spend effort (and money) tackling the real problem.
At its core, teaching children strategies to help them cope with life's challenges- and when and how and who to ask for help if the problem isn't something they can deal with alone- is a good idea. I don't think most people develop these skills without being taught them somehow, either by being taught explicitly or seeing adults modelling healthy coping strategies.
I've seen resilience taught brilliantly in one school I worked at. The foundation to it all was to foster a really supportive, inclusive classroom culture where children felt as safe as possible to take risks and try things without the certainty that they could do it perfectly first time. Then lots of work and support done on what to do if it doesn't work out first time, what could I do if I get stuck, how to manage friendship issues etc. And I think it should go hand in hand with caring for children's mental health, because often they will face challenges that they can't deal with alone.
However, I think this needs to be done as part of normal classroom practice, not teaching 'grit and resilience' as a stand-alone topic. I'm not even sure children need to be TOLD they are being taught grit and resilience. Then it just becomes another target they have to try and meet instead of something learnt at their own pace and developed naturally over time. Learn 5x table, master column addition, become gritty and resilient 🤦🏼♀️