Issues / concerns regarding some of these solutions:
(1) Weight - Amount of weight each option can support varies greatly. If mounting a shelf that could have up 75 lbs in total weight, need to know how many brackets to support that weight safely.
(2) Size of brackets - some of the large solutions like French cleats, etc, seem to be 3 inches in width (top to bottom), thus if the part you are connecting it to is only 2 inches wide, not really an option because part of the bracket would be visible.
(3) Mounting screws - "flush hooks" are nice, but the brackets (both sides) seem designed to be mounted to wood and come only with short wood screws that flush with the beveled holes. However, wood screws would not work well for mounting to dry wall. Thus you need to determine what kind of drywall anchor mount you need to support the weight and hope the screws that come with it, work with the flush hook brackets. The heavier duty the anchor mount, the bulkier the screw head that often will not work with the flush mounts.