As someone who has been the developer of one such app for more than a decade I’ll try to provide the answer based on my experience which was acquired in contact with many such users and also by reading various materials on this topic.
Text-to-speech apps may help those groups in two ways:
- by providing the text in the audio form which may be more accessible to those users
- by providing the text in a more readable form than the original.
First thing to note in driving the conclusion is that both Dyslexia and ADHD are syndromes. As such they don’t work the same for everyone. There is also not a universal answer for everyone and in some cases text-to-speech apps might not be helpful at all, but in my experience success rate is fairly high and it reasonable to take some time to see if this method helps.
Even more important conclusion is that one setup of the text-to-speech app may work for some people with those syndromes, but it may not work for others. As such finding the right solution is a process of testing different setups. It may take some time to find the one that works for you, but when you find it it is more than rewarding.
However not all text-to-speech apps are customizable to the same extent. As such it is a good idea to first find the app that offers the widest range of such options and then to try the find the best configuration in it.
Speech Central was always made with this in mind, and I think that it offers generally the widest set of configuration options. You can see more on this fact sheet that compares features across popular text-to-speech apps (Speech Central, Voice Dream Reader, Speechify).