Each improvement pantheon covers several ressources (God of open sky has for example sheeps, cattle and horses), but God of the Sun is extremly specific. The amount of food is relative high in the earliest stages of the game, but diminish very fast in comparison to other food sources, theres absolutly no late game potential.
None of your points are "wrong", but I will argue with the degrees of your statements and their ultimate conclusion.
1) "God of the Sun is extremely specific". This is technically true, it only affects one resource. But the statement implies "therefore, there isn't enough benefit for a good pantheon". That I will argue. Wheat is one of the most common resources on several maps, and is often found in clusters.
More importantly, its presence is
very obvious. I am not working with hidden resources that may or may not come up (like stone). By the time I am getting a pantheon, I know
exactly how much wheat I am going to be working with, especially if I'm playing Tall and have mapped out most of city sites.
So is the pantheon niche? Absolutely. There are many starts where I would not touch it with a 10 foot pole. But there are starts that it is great for, and when I am choosing my pantheon, I always know whether I am in one of those starts or not. So the specific-ness of the pantheon has no bearing on its power. I use it when its good, and don't use it when its bad.
2) "The amount of food is relative high in the earliest stages of the game, but diminish very fast in comparison to other food sources". Again these statements are true to a point, but not to the degree that you are implying.
First, no other food source approaches wheat with a +3 food added on top. And as sources improve, so does wheat (often even faster). At no point in the game does anything beat the food that a Sun wheat farm can generate.
Ultimately "diminish very fast" is relative. Does it diminish fast compared to other food....no I don't see that at all. If your implying that "food's value diminishes quickly", than that brings me to the 3rd topic.
3) "theres absolutly no late game potential." Again, I agree with the fundamental statement. I commonly advocate that food becomes very weak in the late game, and have used that argument to advocate strengthening buildings like Agribusiness and Medical labs. You are correct that Sun has no real staying power in my late game.
But the statement implies that this is a flaw in the pantheon, and its not. Some pantheons are steady as she goes, some scale later on, and some are all front-loaded. God of the Sun is an unabashed early game kind of pantheon. It gives you an incredible shot in the arm right at the beginning of the game. And like many early game bonuses...its up to you to take advantage of it. If you don't...that's not the fault of the pantheon.
I have used Sun to great effect, and once I am using my early game advantages to manifest a strong middle game, I know longer care if the pantheon begins to taper away. It did its job, I am thankfully for it, and I move on.