Materials
Hard plastics
Rigid plastic has an unglamorous reputation and one significant advantage: it does not absorb the energy it is asked to carry.
ABS is the workhorse here. It is a rigid thermoplastic, moulded to tight tolerances, non porous, easy to finish to a high polish and stable against the mild chemistry of soap and lubricant. It is the same family of material used for keyboard keys and protective helmets, which is a fair indication of how it behaves under stress.
Rigidity and vibration
Vibration travels through a rigid body with very little loss. Push the same motor into a soft silicone shell and a large part of that energy is absorbed by the shell itself, converted into heat and internal movement rather than reaching the surface. This is why a hard shell often feels stronger at the same motor setting, and why a soft outer layer over a rigid core is a common way to get both a pleasant surface and an efficient transfer.
The same physics explains a complaint that comes up often: an object that feels powerful in the hand can feel weak in use, because the hand is a poor test surface. Pressing an object against a soft surface loads the motor differently and changes both the frequency and the amplitude that actually arrive.
Seams, gaps and finish
Injection moulded parts are made in halves, and the join between them is visible as a seam. A well finished seam is smooth to a fingernail. A rough one is a snag risk and a cleaning problem, since a groove holds what a smooth surface does not. Run a nail along the seam of any rigid object before first use, and again after a few months, because a hairline crack starts at exactly that line.
Coatings
Rigid plastics are often given a soft touch coating, a thin sprayed layer that feels velvety. Those coatings are the least durable part of the object. Oils, solvents and some lubricants break them down, and once a coating starts to lift it becomes sticky and traps dirt. A plain polished shell with no coating will look worse in a photograph and outlast the coated version comfortably.
Cleaning
Warm water, mild unscented soap, a soft cloth and thorough drying is the whole routine. Rigid plastic tolerates none of the heat treatments silicone allows, since ABS softens well below boiling and will warp. Alcohol wipes are acceptable on a plain shell and a bad idea on a coated one. The cleaning page sets out the method by material, including the sequence that keeps a charging port dry.
Rigid plastic ages differently from silicone. It does not become sticky or porous, but it does become brittle after years of ultraviolet exposure, which is an argument for storing anything in a drawer rather than on a windowsill.