Speaking with my joinery/carpentry - design, detailing and specification hat on ..... from what I have learned over the last 30 years of practice - from the joinery firms and cabinet makers I have worked with ....
My two penn'orth would be to advise sealing whatever substrate is used to limit any tendency towards movement via absorption or humidity - and make sure that the sealant is not microporous. I would also observe that if the intent is to bond to one face (particularly if you are bonding a material which might tend towards shrinkage on drying), then it would be wise to either counter-balance the other side, use a thickness which would make such bonding all but immaterial, or brace to achieve the same effect.
As a further observation, my experience with thinnish non exterior grade plywoods is that the top face will absorb water and expand, but that the glues prevent significant penetration to the lower layers so a marked bowing occurs. This would be another reason to avoid waterborne products in this instance.
Exterior grade and moisture resistant MDF are both available, are stable and are not subject to this kind of bowing. However MDF does have a tendency to 'flow' due to its internal structure and so if used horizontally will almost always sag over time (dependent upon the thickness used and centres of support). It will do this due to its self weight without loading.
Beware of block boards. In the old days blockboard was often lipped, veneered and used for shelving ... but the BS has changed and the internal bonding is much cheaper and less effective, meaning it will now sag quite alarmingly.
Having said that ... if something has been working for you over time, then you must be doing something right, so why change?
I observe the guidance above because I can be sued if it goes wrong and so better to be safe rather than sorry.