Chime in with my 2 cents...
I think hard acrylic isn't necessarily uncomfortable... after all, for years that's all they were made out of. The question here is more about isolation, as I understood it. Hard materials will separate slightly from your ears when you open your mouth, because the facial muscles will effect your ears shape slightly. When you go and make custom molds, they ask you to open your mouth slightly, so they can get a mold that'll hold when you open your mouth, and maybe a little tight when your mouths are closed. Of course, if you open your mouth really wide or yawn, they'll still lose their seal temporarily.
If I had to choose between hard acrylic and universal fit plugs (tri-flange, foamies, whatever), I would think that hard acrylic is still better.
However, soft materials will flex and bend as your ear shape changes slightly, so they can hold their seal much better even when you're talking... and of course, singing (after all, they were made for that purpose).
I wouldn't also think of hard acrylic as being anymore "rugged" than soft materials of any kind. Think about it, if you drop hard acrylic on the floor... it'll scratch and possibly crack. If you drop the soft materials, they'll just absorb the impact.