In my opinion, Abe-Hicks is actually more similar to Neville than people realize. I haven't read A Course In Miracles or Marianne Williamson so I can't speak on that (though before I got into LoA I used to religiously read Williamson's "your deepest fear" quote every morning after waking up and I believe doing that helped me reprogram my subconscious mind to make my life more awesome).
How is Abe-Hicks similar to Neville? First, Abe-Hicks says you are Source Energy (God). Secondly, your only job is to accept that what you want is already done for you. To help you accept that, get into the feeling-place of your desire being yours now. It's very straightforward, it's the same idea as getting into the wish fulfilled. By the way, these ideas were talked about in books before Neville Goddard like Florence Scovel Shinn's and Genevieve Behrend's work, among others.
How is Abe-Hicks different? Obviously, terminology is the big one. But Abe-Hicks also preaches working with your feelings. When you're feeling very negative, it's really hard to feel the wish fulfilled, at least that has been my experience. So Abe-Hicks says to find relief first, use better-feeling thoughts to change how you feel and then it's easier to feel better and that makes it easier to go into the state of the wish fulfilled. I know this might seem unnecessary to some but after only following Neville for almost a year and becoming more depressed with no results, these aspects of the Abe-Hicks teachings really helped me make my life better and then they helped make it easier to feel the wish fulfilled when I visualized.
"Going general" is very misunderstood. It does not mean to give up on a specific job, specific person, or any other specific desire just to pursue a generic desire. Instead, going general means to make your thoughts more general but still evoke the feeling of the wish fulfilled, or evoke relief that helps you more easily transition into the feeling of the wish fulfilled.
For example, if I say "Mary loves me" after a rejection that still stings, then it feels bad, it doesn't feel real, it feels fake and it reminds me of all the problems between us. So do I keep repeating it and feeling like crap or should I drop that thought (not the desire) and find a different thought that makes it easier to feel the wish fulfilled? Choosing the latter option, I'll go general - I know I can heal, I know life can be wonderful either way, I know I can be attractive, I remember Mary enjoys my company, I remember Mary does care about me, I know she cares about me, I know that I can be confident in myself....I repeat one thought and bask in the feeling, then repeat the next if it feels truthful and good and bask in the feeling of it, going up and up, these more general thoughts helps me feel more relief, helps me feel better, more confidence, more peace, more fulfillment, The more I let the feeling grow then the easier it is to be more specific and say "I am loved" and to even feel it from her and to really mean it and to really feel it real. From general to specific - the process is easier, you might even find that repeating the specific thought isn't necessary because the feeling of the wish fulfilled has already been evoked by a more general thought. Many years ago, this is how I connected again with a SP, this is how I practiced self-love, indulging positive self-talk and positive activities that made me feel more love and confidence in myself and this made it so much easier to live in a state of being loved, to become aware of myself being loved.
And speaking of self-love - Agnes Vivverelli talks about Abe-Hicks too. She doesn't focus on it that much but when I see her videos I can see how she sneaks some Abe-Hicks in there, especially in the Neville videos. Neville never talked about self-love (even though I think you can make a case that his teachings support self-love). But Abe-Hicks recommends self-love.
Neville also has examples of "going general". If you want to manifest multiple things at once with one scene, he recommended to imagine yourself being ecstatic for no reason, repeating to yourself "Isn't it wonderful?" The lullaby method where you repeat "Isn't it wonderful!" or "Thank you Father!" or "It is done!" like a lullaby to feel the wish fulfilled is also another example. Joseph Murphy, a New Thought author similar to Neville, also gave an example of a man who repeated an affirmation for wealth but it felt like a lie. So he recommended the man to change the affirmation in a more general way but to still evoke the specific feeling that the wealth is his.
Now let's talk about some big misunderstandings of what Abe-Hicks teaches. You DO NOT have to be happy literally 24/7. Abe-Hicks even says this won't happen because feeling bad is a part of life and actually part of your expansion. It's not about suppressing your feelings and forcing yourself to be happy. It's about soothing yourself where you are, choosing the thoughts and activities that help you feel better, and as you guide yourself into feeling better, it's so much easier to feel the wish fulfilled when you visualize. This is the biggest and I think the most dangerous misunderstanding I see.
The most interesting rebuttal to Abe-Hicks teachings is that it emphasizes consciously feeling better instead of subconscious reprogram like Neville focuses on. But Neville did talk about mental diets and positive self-talk during the day, implying conscious methods are effective. I'll admit this argument was a big reason I avoided Abe-Hicks but I regret that now, it just helped me so much. Plus, strong feeling during the day can still get the job done in my experience, Neville himself said that going to sleep doing the technique is not the only way.
There's one word Neville emphasizes that helped me connect the dot with why I think the Abe-Hicks teachings helped. Awareness. Neville emphasizes awareness of self, to be aware of yourself being what you wish to be. At first, it might seem like nonsense when Abe-Hicks talk about feelings and feeling better. But feeling better just makes it so much easier to sense the change in awareness, to be aware of yourself being more of what you want to be.