Keymod uses an undercut flush mount that maximizes strength for being lightweight and having minimum bulk. Someone using niche market high end mounts, who have no need or desire to stack numerous accessories against each other, will having a very sleek and light forend with ample internal clearance for suppressors, gas blocks and such. You will have no issues with lack of strength, it will be sleek and bombproof.
If on the otherhand, you have a need or want to fit, or retrofit any number of the many plastic Magpul accessories, or you are military and need bulk orders, that work with retrofitted stuff, then the bulkier mounting system of the mlok works, whereas the keymod will not. The mlok is cheaper and easier to manufacture in bulk, and therefore a big winner for Magpul.
Using my first choice favorite as example , KAC, they make both, with both being top shelf. Pick your poison. If you are a fan of a great keymod forend, likely you will shy away from mlok. If you need mlok for your stuff to work, then you will not be looking at keymod.
