apperantly it is not, from reflected EPiServer.Find.TypeSearchExtensions class:

namespace EPiServer.Find
{
  public static class TypeSearchExtensions
  {
    private static HashSet numericFieldTypes = new HashSet()
    {
      typeof (int),
      typeof (int?),
      typeof (double),
      typeof (double?),
      typeof (short),
      typeof (short?),
      typeof (long),
      typeof (long?),
      typeof (float),
      typeof (float?)
    };

..........

Is there some particular reason for decimal not being numeric type in EPiServer's Find viewpoint?

Decimal wouldn't work, You have to change the type to double, int, float or short
Or by parsing may work
RangeFacetFor(x => (double)x.TestDescimal)

/K

That is perfectly clear, question is: why EPiServer decided not to include decimal as numeric type?