I leave too little criticism, either complimentary of the work done or suggestions for improvement.   Yet, I do comment fairly regularly.

When I do provide constructive criticism, it will be a simple suggestion generally dealing with photography technique or composition and not so much with postwork beyond basics.   Site mail works fine also and does keep it private if you feel better about that.

We sometimes forget that positive criticism is also part of growth and reinforcement of good work done.   It might be easier to be critical in offering improvement than is offering genuine sounding recognition of good work.  A "wow" won't do it..not generally.