Greetings from Nebraska.
I am pretty sure Dehn0045 is correct. Before wood stabilization became widespread there were a lot of folks out there putting together chambers for de-gassing paints and such, and most of them used 1/2-inch thick Plexiglas(R) acrylic lids which are only marginally thick enough. Combined with the practice of causing stress points by drilling holes in them for gauges and such, and as Dehn0045 stated, the stabilizing chemistry would start to react with them resulting in a catastrophic failure. (I personally experienced such a failure even with a 3/4-inch thick plexiglass lid - it was an impressive implosion and scared the living crap out of me)!
So, when wood stabilization took off the blame for catastrophic failures was widely put on the chemical interaction between stabilizing chemicals and plexiglass and rather than substantially increasing the thickness of the lid or changing the material to Lexan(R) polycarbonate (better) or glass (best) and substantially increasing the cost, many simply added the "Not for Wood Stabilizing" warning. Over time it may have simply become a boilerplate disclaimer for anything that comes with an acrylic lid.
After my implosion, I used a step drill and mounted the inlet/outlet and gauge through the stainless steel rather than going through the lid and switched to 3/4-inch Lexan(R) polycarbonate for the lid, but after several uses it looked like there might be some crazing on the surface. Having a strong motivation to never, never ever, experience an implosion in my shop again, I finally replaced the lid with one made of glass and have been stabilizing relatively anxiety free ever since.
Although many have had success with less than optimal configurations, my recommendation is to 1) Avoid plexiglass and opt for polycarbonate or glass if possible; 2) Opt for the thickest lid material you can find within reason; and 3) Avoid lids that have holes drilled through them if possible.
Dave
PS Plexiglass has become the genericized trademark for acrylic products such as Plexiglas(R), Lucite(R), Acrylite(R), and Perspex(R). Polycarbonate brands include Lexan(R), Hyzod(R), Markrolon(R), Susta(R), Tuffak(R), and Tecanat(TM).
