The main story is I think nice. Personally I don't like the designs of Clannad nor the voice actors and pretty much the majority of the side characters. But the main story works because it tackles subjects you don't see too often done in anime especially in moe anime. I think there is real showing of growth throughout as Tomoya is tasked with more responsibility as someone who has to protect his family. It's done in a more realist way of providing for them by getting a job, and understanding your significant others and your own emotions to help each other grow. It means letting go of baggage, dealing with tragedy in a healthy way, and making tough decisions and living with them. One part of his growth is how he gets a better understanding of his own father. He understands that adults aren't perfect by becoming one himself and appreciates what his father did and sacrificed to help him become who he is now. This is shown in Tomoya far more than Nagisa however where she feels perfect. I don't think she lowers the story or she's bad but that she feels more like a foil for Tomoya's growth than an actual character at times.
Ultimately though, a story like this is reliant on how much you connect to the characters and in that sense it didn't completely succeed in making me buy into them. Some moments and situations are sad regardless of the characters. A kid getting hit by a car and a mother being devastated is a sad thing by itself but what a story needs to do is go beyond the simple sadness of the scenario. In SKET Dance, a scenario similar to this happens and what adds to it is the irony of the whole thing and the way it connects to characters that you've already gotten attached to (Admittedly the very final ending of this Clannad also has some of that irony). Personally I still didn't fully get attached to the characters as I would in other sentimental shows though I could see how if you did, what happens would be even more impactful. I think the main storyline is a 7/10 or 6/10 and the mileage on that will change depending of how much you buy into the characters.
As for the ending, basically the ending itself is I think correct in that it's "destined" but at the same time the way it's done sort of feels like it's forced with somewhat irrational decisions from the main character. It feels a little bit more manufactured and the alternate storylines are introduced right afterwards adding confusion as to the intent of showing it. I think some people would be impacted by the immediate contrast differently. Some maybe would be even deeper in their feels but to me it almost seemed to make frivolous what happened right before. Maybe it'd be redundant or not as poetic but I'd like them to go further with what happened at the end.
Also finally, the illusory world isn't too bad. I think the whole concept and what we discover about it including who the girl and robot are adds a dimension to the themes of family, loss, change and memory that are explored or hinted throughout the series.
