have you ever thought about the possibility that intelligence tests are actually not as reliable as we think? i think we are relying too much on pseudoscience and too little on our human understanding of each other. I would suggest you read Gould, S. J. (1981) The mismeasure of man, (New York, W. W. Norton & Co.) it is a fascinating book which explores the negative aspects of scientific measuring of intellectual abilities.
Who thinks they are reliable? It depends on one's definitiiion of intelligence. And there are many ethical issues as to how their results are used. For example in the field of dyslexia it is now acknowledged that dyslexia can exist at any level of the present whatever the ability of the individual.
After years of working in the field, I still find that significant delays of functional development against developmental milestones the most important factor in determining should you intervene or not. Every early effort to support a child with delays will be of benefit and is unlikely to do harm. After all, what earthly use is an IQ score to a 3 year old? Time and effort should rather be focused on ensuring the ability of the child to learn across all domains before trying to measure IQ. Until all developmental opportunity is ensured, no test will be reliable.