There are two basic issues with the Valentino/Velox/GT series of mechs - and Huret Svelto, Roto. one version of the Zeus Alfa, maybe ZZR made one...

The first is the placement of the upper jockey wheel at the lower knuckle pivot; any with this design force that jockey wheel upwards when the sprocket teeth it is aiming for are getting lower. A mech with an offset pair of jockey wheels can be arranged so that as the cage rotates to unwrap chain for the larger gears, it moves downward a bit. (The Valentino has this.)

The second is the poor parallelogram pivot design. They are single-sided, and the steel of the knuckles or the arms (or both) is too weak - often it becomes bent, whereupon you get stiff movement - or sloppy, when enough grit gets in the gaps after you oil it trying to loosen it up. The only successful version of this design I know of is the Jubilee, I think it's just that those arms are a thick enough section that they don't bend (and possibly superior manufacturing tolerances). I suspect the Valentino Super other Zeus Alfa might be worth trying, their arms are U-shaped and so those pivots are not side-loaded, and they have offset cages.