everytime i try and multimeld shields i see my guy using his hammer
And every time my character had converted an item he's materialized a swirling vortex of light between his hands. Just because the animation doesn't show the described special items in use doesn't mean that's not the intent.

One DoH being able to meld any item wouldn't be a "screw you" to DoHs at all. What's so distinguishing about being able to meld only certain items?

The distinguishing factor of DoH is the items which they make.

or to put it this way. how would a blacksmith know how to stitch materia to my robe. or how would a weaver know how to weld materia to my sword?
That was the point of showing the descriptions of the key items. The process has nothing to do with the craft it requires.

In-game, there's a book called "Life, Materia and Everything" says:

Section I - Required Ingredients
Melding a piece of materia to an item of equipment cannot be achieved via conventional means─a compatible catalyst is required. Catalysts serve to stabilize materia, allowing them to merge with host items.※ Catalysts are consumed during the melding process. Not all catalysts are suitable for use with all materia. Similarly, specific materia can only be attached to certain types of equipment.※ The mechanism by which materia appears to select or reject its mate is the subject of a separate study.
A weaver doesn't stitch materia onto a robe. They use the melder to cause the catalyst to magically fuse the materia to a robe.

Why is it that it shows a weaver using their thread and needle and not the melder? Because it was an animation already on the game accessible to a production team behind schedule. Which likely also I'd the reason that despite the assimilator being a "complicated-looking device" all we see is that swirling vortex of light between our character's hands.

Melding doesn't distinguish one craft from another. The class restriction is arbitrary to the game's explanation of how the process is completed.

But to those steadfast in a position against one class melding, I propose this question.

Why is it okay for one aspect of the process -- converting -- to be done by anyone at any level where as melding has such stringent requirements when they both use unique tools to magically alter the state of a crafted item?