It's not really an opinion so much as fact. Labor unions have proven to be beneficial only for the members of the union and politicians. If you had ever done business around them, you'd agree.
That would be ridiculous, since that is not the argument that needs to be made. Instead, you want a logical argument as to why the concept of collective bargaining in the public sector is a bad one. I won't provide one since they are readily available if you have an ounce of curiosity. I'll just leave this:
“It is impossible to bargain collectively with the government.” - George Meany
Please, tell me more.