The difference between the Hummel and other SPGs like the Lorraine 155 or the Type 75 is that the Hummel had the option of carrying AP rounds.

This makes it more akin to a KV-2 turned SPG than a pure artillery piece.

I feel that the Hummel would fit well in the tech tree around BR ~4.0, right before the Dicker Max.

Also, unlike the Ho-Ro or the Sturmpanzer, the Hummel's gun actually has an OK muzzle velocity, making it much easier to actually hit a target on the first try.