No too much load, really. We (Bitbucket) already have pre-receive hooks for a handful of other things. The trick would be defining the rules properly to have a reasonably low false negative rate while avoiding work inhibiting false positives (or allow for a mechanism to override it with, say, a force push).

Of course, 93% of our repositories are private so this feature may not be exceedingly useful to our customers vs other things we could be spending our time on.

Edit: I shouldn't have said not useful, rather, comparatively there may be more value in us pursuing other work first. E.g., provide a mechanism for 3rd party pre-receive hooks via our add-on system.