As you mention an idea of serious sustainability -- which is surely an important concept at this time -- what is it about holidaying or even living in yurts that you find non sustainable?
Essentially my point is that this sort of 'yurt living' (in an environmentalist context, not a holidaying context) is promoted as part of the, say, permaculture lifestyle - let's all get a few acres of land, grow our own potatoes and lettuce, and make do with whatever primitive methods we had 100 years ago. Completely ignoring aggregate level ecosystem service accounting, or even the basic observation that for most people living in a tent is not acceptable. So it's more a remark on the lifestyle view on sustainability that is associated with 'living in a yurt' than actually living in a yurt (or similar 'alternative dwelling') itself. 'Living in a yurt' isn't just 'I don't live in a house'; it means 'living without running water, sewer systems, insulation or efficient heating in very low population density systems, with a focus on self-sufficiency, in 'connection' with the land and in small-community social contexts.
The glamour and possibilities of an intimate experience with natural materials and settings is part and parcel of cultural and technological development towards the goal of mass coexistence with natural systems, through a variety of places and architectures.
Even ignoring the benefit of investing attention and sense of identity in biophillic aesthetic and ideology, the typical enthusiast of yurt living or similar is at this stage no less ecologically viable than the rest of us - who are currently occupying space and resources and polluting directly and indirectly plenty.
Rural biodegradeable cabin dwellers might not be able to live quite as eco-lightly as urban dwellers might be able someday in a specially arranged ecologically optimised municipality, but such optimised municipalities dont exist yet.
Experience reveals we dont all benefit from living with plumbed water and toilets -all of the time. Poverty is more complex than the presence or absence of such facilities.
The history and development of permaculture technology does not deserve special cynicism and is never criticised with representative data.
I've seen the glamping sites with yurts in the lake district before, but I assumed they're few and far between. Guess I was wrong.
https://coolcamping.com/glamping/yurts