Is the unbounded fan-in model realistic? Does the unbounded fan-in circuit model apply in "practical" settings? In other words, are there real-world realisable computers with unbounded fan-in gates?

As I understand, standard silicon ASICs are made of so-called cells where the number of input signals in the largest cells is small (e.g. never exceeds a fan-in of 100).
Don't forget that even though the fan-in is unbounded, the number of gates is polynomially bounded in the number of variables $n$ (in the definition of $\mathsf{AC}$ for instance) .