Hugely complex question. It has been considered and reconsidered many times. There is the neutral theory, which states that the vast overwhelming majority of mutations have no effect on the phenotype. The nearly neutral theory and the classical theory. Neutral theory is a big subject.
On indels, these can be "neutral" and reversing depends how much selection pressure they are under. Microsatellites do "reverse".
Essentially, this is a massive question and its better to consider a single species as an example for a single type of mutation
See the comment by @MaximillianPress. That is a good answer, the step-wise mutation model is not a bad model for (micro satellite indels)