There are lazy people that work in every job. Some have the good fortune to land a union job - which fortunately for them and to the detriment of others does usually make it harder for them to be fired. Conversely the people who do work hard are rewarded at a rate that is more commensurate to their skill level and commitment and receive the kind of improved job security and work environment they are deserving of.
Most often though, if a person is truly deserving of termination, union or no union, they will be fired in due time.