What are the limitations for using hydraulic vs. mechanical snubbers? Displacements? Frequencies?

Typically, mechanical snubbers are not suitablefor out door applications. Secondarily, hydraulic snubbers have more options asfar as the available strokes. Frequency response and displacement wise, thereis no difference between hydro/mechanical snubbers.

Mechanical Snubber Assemblies Custom Designed for an Oil Refinery in California
Mechanical Snubber

Hydraulic Snubber with an Overall Stroke of 6 Inch
Hydraulic Snubber