Winemaking is like any fermentation or curd making. If you add too much jaman (curd starter culture), then your milk will turn into curd faster, but there will be no other perceivable change in your final product. However, if you don't add enough starter culture, then the milk will spoil. Similarly, wines require a dosage of yeast. If the active yeast cell count is too low, you will be prone to spoilage. However, adding too much yeast will only lead to faster fermentation and no other change. The yeast will finish their job fast, settle at the bottom, and will be raked off.
Adding too much yeast nutrients or DAP is serious. It is a chemical and usage of more than 1gm per liter is banned in the USA.