This is relatively new to me, so thank you for posting about it. I did want to remark a couple things in reply to what you posted:

• The article says that the Miyawaki method specifies that it uses native species, so it would seem that any use of non-native species is the “fault” of whoever is choosing the plants. That said, as you noted, this method doesn’t quite seem to address site-specific conditions, such as soil types and water availability.
• The article also mentions that these gardens (like the article, I find that I hesitate to call them “forests” for various reasons, the biggest being their small size) require “zero maintenance after planting”, which I personally doubt (at the very least it should require some watering to get the plants situated), but if it’s a low-maintenance thing using native species correctly chosen to fit the site, then there shouldn’t be much reliance on irrigation.

Of course these points are in addition to the concerns over exactly what the broader ecosystem benefits of such an approach are. For example, the biodiversity profile of soil microbes and fungi – which (far as I know) aren’t intentionally chosen in this method as it currently stands – might be different between a site that’s naturally regenerated and a site in the same local biome that’s been quick-reforested this way. And there’s also open questions about how useful such spaces are to animals. Mind you, I’m not saying that the answers to these questions are negative – they may very well be positive. But they’re just potential points of future research. (Also, use by animals might be site-dependent, so if this method proves to be useful in this way it may be a decent method for re-creating wildlife corridors.)

Still, this method seems effective in delivering at least some ecosystem services, namely carbon sequestration, canopy shade, and various other benefits of simply having a stand of trees around. So it’s not useless. And given its relative “disconnection” from natural regeneration, it might be more useful for small-scale “urban afforestation” – when you wanna just get a stand of trees planted and make their own at least somewhat self-maintaining mini-ecosystem, especially on land that’s likely never gonna be allowed to undergo a natural succession process (at least not with native plants), such as small spaces in cities and suburbs. For example, here in south Florida there’s a matrix of dry land and water bodies (west of the Atlantic Coastal Ridge) artificially created from what was once part of the Everglades. There is no truly native upland habitat here, but…what can we humans do with it now that we’ve built houses and roads and stuff on it? If we plonk down a forest here, it’s not like it’s going back to how the marshland it used to be, but that doesn’t mean we shouldn’t plonk down a forest (with appropriate species choices, of course).

Speaking of wetlands, I’m also curious whether such an approach can be adapted to wetland restoration. From what I understand, processes like peat buildup are very slow, but are there ways to accelerate this with minimal ecosystem disruption, maybe even if restricted to artifically-created wetland systems?