Spider is slightly higher R at moderate delta-Ts than dense-packed cellulose, but isn't likely to impede infiltration nearly as well, and will have significantly more convective-loop losses at high delta-Ts than dense-packed cellulose (but far less than low-density blown fiberglass).

I'm highly doubtful that Spider blocks radiated heat as well as cellulose. Got any references to back that up? Glass is somewhat translucent to radiated heat, but cellulose fiber is quite opaque. Most manufacturers include additives to reduce IR translucency, but SFAIK there haven't been any recent breakthroughs on that front.

At 2"+ depths Spider is rated as a thermal barrier against ignition for foam insulations without needing to use gypsum or plywood, adding another ~ R8 to the stackup.