Some builders are good.

The comments about bad service and cheating by builders are valid, however is worth mentioning that there are many tradespeople around who provide a good service to their clients. That is evidenced by the number of local repeat clients that they have.
As a new client it is worthwhile asking a builder if they have a list of their regular repeat clients. If they are willing to present one, with some who would not mind being questioned, that is a good indication. If not, that is also a good indication of something.
A person trading is usually keen to build up a list of people who are willing to refer to them. If they do not work in that way, conclusions can be made.

I worked for many years in a company which supplied services to builders tradesmen. Some of the stories, although possibly exagerated, were sad. They often seemed to result from the client asking works which were not practical, or possible, to do in the time, cost, or manner which was requested. After a refusal, a client would then threaten to get another builder - so the first one said that "he would do the best he could". Then the problems started!

Memories are those of exceptions.

It is the same as many things. One's memories are of exceptions. A person can drive all week passing thousands of drivers in both directions at close distances - only remembering the one or two who caused agrevation by their driving methods, all others are forgotten. However the few dangerous events are memorised well - when, where, the colour of cars, distances, etc.

Many problems that clients have are caused by them not appreciating what work is really needed when the agreement is made. By agreeing what will happen, regarding price and schedule, for extra works a great deal of conflict will be avoided.