I've never had any problems screwing into ply so long as you have noted, pre drill first. Now, screwing into MDF, this is something totally different. MDF doesn't really screws that well. Ply, you can get away with it and still have the strength needed. I should note, this also depends upon what type of ply you're using. Birch, cabinet grade, you know, the good stuff
, there aren't issues with it. The cheaper grades, have voids and this is where you'll run into trouble. Remember, ply gets it's strength from the multi-layers it has, which, is also, cross grained, just adds more strength.
Just my $0.02 worth.
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