Online IQ tests are easy to take and rely on information gathered by experts. The question to ask is are they accurate? Depending on what time of day and the mood of the person taking the test will determine the outcome. Someone taking the test in the morning and that has drunk 2 cups of coffee will test differently than someone who hasn't.
Also, someone who takes supplements that enhances their focus and concentration such as Matcha tea will do be better on an IQ test. These factors come into play, but even if a person does not do any of these things the testing can be different every time. Generally speaking, the test will have similar results each time the test is taken. Most of these IQ tests online are developed by academics who do know their stuff.
As mentioned earlier, the mood of the person has a great deal to do with the results. That's why to get a test that's close to being accurate the test needs to be done several times to get close to an accurate iq test result. Take for instance a person in a bad mood who is not receptive enough to score well on a test. On the other hand, someone who is in a good mood will score higher on an IQ test.
An IQ test online or otherwise is good to find out a person's IQ in general. However, it should not be the end-all of determining a person's intelligence. In some academic circles, the IQ test is being done away with in favor of more accurate means of measuring the intelligence of a person.