When you move to a hydraulic thumb there are important design considerations that are often overlooked.
On any control valve, for example a loader or a hoe, the main valve incorporates a relief valve to limit the amount of weight/force which can be lifted. This relief valve is only in the hydraulic circuit when the control lever is moved out of its neutral position. Once the control lever is back in the neutral position the hydraulic fluid is locked in the closed circuit.
An example to explain what I mean: If you use a loader to pick up a full bucket which it is just managing to lift because the relief valve is saying enough. Now you get off the tractor leaving the bucket raised and decide to shovel more material into the bucket. The relief valve is no longer in the circuit protecting the hoses, cylinders and valves. By adding more material to the bucket, the bucket and lift arm cylinders are having to support more weight and in doing so they become high pressure hydraulic pumps and expose parts of the hydraulic system to pressures which can fail components.
With a hydraulic thumb you have two hydraulic circuits forcing against each other. By virtue of the linkages involved, either the bucket or thumb will have greater leverage over the other circuit and will have the effect of greatly magnifying the hydraulic pressures in one of the circuits.
I am not an expert on hoes or excavators but have read on this and other forums of owners finding they have bent cylinders and blown hoses.
This short video by Messicks demonstrates how easy it is to over pressure a hydraulic system when existing components become pumps.
Messicks
If you want a hydraulic thumb then don't just add a cylinder to the existing mechanical thumb but find a knowledgeable company to give you a reliable installation.
Dave