The major cons of the UAW are the same as the cons of any major labor union. That is, they drive up costs and make it harder for unionized companies to be competitive.
The main point of a labor union is to make life easier for its members. Labor unions try to ensure that their members will not be abused by their employers. However, they also try to go beyond that and to ensure that their members will have the best possible compensation packages, the most comfortable possible working conditions, and the most secure jobs. This can lead unions to push companies to enter into contracts that turn out to be unworkable. This has been the case with the UAW.
In the past, when there was little foreign competition, the UAW won contracts with American automakers that gave their members great pay and benefits. Today, those contracts weigh down American automakers. The costs associated with those contracts make it very hard for them to compete with foreign car companies that did not give unions such lucrative contracts.
Thus, to the extent that there is a con about the UAW, it is that they were too successful in times when American car-making was booming. They negotiated contracts that later turned out to be harmful to the automakers.
Im not sure how could those contracts harmful to automakers?
Could you be more specific of how are those contracts weigh down American automakers?