Even though they work somewhat, they rely on generation of arrays and sometimes Data Validation doesn't work the way it should. My feedback (actually complaint) to Microsoft is that there should be such functions to begin without using VBA with given how often they are used.
ISALPHA() - check if given text string has only alphabets A-z, a-z
ISUPPERALPHA() - check if given text string has only upper case alphabets A-Z
ISLOWERALPHA() - check if given text string has only lower case alphabets a-z
ISTEXT() doesn't work for my case because it can include special characters (e.g.!@#%^&*) and numbers mixed with alphabets.
Even if I could declare a UDF, it probably will not work in Validation.