In a medium saucepan over medium heat, stir together the rhubarb, honey, wine and balsamic vinegar. Bring to a boil, then lower heat to a simmer, and cook until the rhubarb has softened, about 5 minutes. Add the red pepper flakes, cayenne pepper (add in increments if you are sensitive to spice), shallots and garlic and let it cook for about 5-7 minutes more. Transfer to a blender and blend until smooth. An immersion blender also works. Pass the sauce through a sieve and return to the saucepan and return to the heat.
Season the sauce with salt and pepper, and continue simmering the sauce until it has thickened, about 10 minutes. Adjust seasoning if necessary. Remove from heat and let it cool; it will thicken as it sits. The sauce may be used right away or stored in an airtight container in the refrigerator after cooling completely.