Homasote has for years been accused of being unstable in the presence of moisture. Yet others have used it for years with never a problem. I read a report where a piece of Homasote was immersed overnite. It swelled while in the water, but resumed its shape when dry. I believe that part of the problem may be substitution of Homasote look-alikes by lumber yards. That happened to me because nobody had real Homasote in my area, so I use a similar substitute whose performance in the presence of humidity may vary from the real stuff. FWIW, I had no problems with my handlaid track on the Homasote substitute for 7 years and moves from coastal Oregon, to pan-handle Florida to non-air-conditioned house in Miami, FL.
FWIW, your wood benchwork also expands and contracts as it moisture content changes. Some of the problems attributed to Homasote may in fact be problems with the structure underneath. There are 3 ways I know of to combat moisture-induce expansion and contraction:
1) control the humidity in the layout room. Note this makes the room more comfortable for humans, too.
2) paint or seal the Homasote or wood. This significantly delays changes in moisture content, usually enough to prevent problems, but does not totally eliminate them. You need to paint or seal all surfaces, not just the top or visible.
3) Use a material that does not respond to humidity either along with wood/Homasote, or in place of. Foam glued to wood or Homasote will help constrain expansion/contraction.
4) Use a combination of the above techniques.
On my next layout, I will probably use all 3 techniques. Homasote is my preferred roadbed for handlaid track. I will use a sandwich of 1/4 inch plywood and foam for my subroadbed to reduce weight on the shelf layout. To keep family peace, my layout will hang on the walls in the living area of the house - either a spare bedroom or family room. There will likely be some humidity control. And I will likely seal the Homasote as a precaution. Don't know if the sealing is overkill, but the thought of a $100+ kit-built and modified locomotive seeking its lowest point of equilibrium (aka the floor) bothers me a lot! I'm a whole lot less interested in risk in my OF&S (Old, Fat, and Slow) years.
yours in wood product construction