One of the biggest challenges with a design is going to be putting the weight where you want it, though. It impacts some shapes & weight distributions more than others, and of course there are other design constraints you may have.
Here are some CAD sketches that hopefully illustrate what I mean.
Full disclosure: These were pretty quick and I haven’t actually had any designs prototyped. But hopefully the numbers can tell a story.
The constraint of where you can put weight is most severe for V-ish yoyos with small rims. Let’s say you want it to sort of look like this, with as much rim weight as you can manage, and ~63-64g assembled:
In 7075 Al, if you want to get it up to ~60g unassembled without changing the catch zone, you have to bring the rim down into the cup pretty far. This rim is > 7mm thick around the edge:
As a side effect, the rim-weightedness is decreased somewhat. Take note of the mass moment of inertia around Ixx: 14,050 g mm^2.
Meanwhile, in a 7075+SS design:
15g SS ring, 10,930 g mm^2
14.82g Al cup, 5227 g mm^2
~60g total mass, 16,157 g mm^2. That’s appreciably higher than the mono-metal version, which had to put some of the material closer to the axle. This doesn’t mean it’s better (you’d have to prototype it!) but it’s different, and not something you can achieve with this same design in monometal.
For O-shapes and other shapes with broad rims it’s a lot easier to push weight out to the edge, of course. I think this is one of the reasons why you see inverse-round designs that have a broad-ish rounded rim after the inverse part.
Another consideration is that titanium (as noted earlier) is ~2x the density of 7075, which makes it better for pushing weight around in a monometal, and 6061 is less dense, which makes it more difficult to push weight around.