Regarding blown-in insulation, several people, for example, have advised us against it. You can't get it evenly distributed in there, and then you end up with thermal bridges.
1. If done by a professional, then you have—provided the cavity is accessible everywhere—insulation without gaps. Blown-in insulation is no longer exotic today but a very common technique. However, mistakes and mishaps can always happen, regardless of the trade.
2. And even if one spot is missed (unlikely), you still have 98% of the wall insulated, which is 98% more than without insulation. Energetically still simply good.
3. Who exactly are the "several"?
Friends of ours recently stripped one of their walls for an extension. It was visible that it’s not a "clean" cavity, and there was a lot of debris and crumbs in the lower part of the wall.
Then so be it. To forego affordable insulation because of a little debris and crumbs in the lower area of a wall (which, by the way, you can see beforehand if you look into the wall)? Because what is the alternative? Exactly, no insulation at all. And that is simply stupid.