The disciples' unbelief in Matthew 17:20 was not a disbelief in God's power to produce deliverance. It was a "natural" kind of unbelief that came from a heart that was more sensitive to what it saw than to what it believed. Jesus said that a very small amount of faith (a "mustard seed") is enough to remove a mountain, if no unbelief is present to hinder it.

Jesus was disappointed that His disciples had not been able to cast this devil out. Most people would have had no criticism of these disciples.  It wasn't normal to have power over evil spirits. Jesus had already given these disciples power and authority over all devils. These disciples had used this power and authority successfully on previous occasions, making them more accountable than others. They weren't just normal men any longer. They had a supernatural ability and calling.

We also receive power from on high when the Holy Ghost comes upon us, and it becomes our normal Christian service to use this power to witness to others.

We can always look around and find plenty of Christians who are not using this power, but 2 Corinthians 10:12 says that comparing ourselves among ourselves is not wise. 

Most Christians are living so far below what God intended for them that when someone begins to live a normal Christian life (according to God's standard), then everyone thinks he's abnormal. We will be successful when we not only build our faith but destroy our doubts.