We all know Monty and Ben have different last names.
And my favorite explanation as to why is that Monty took his wife's name when they married to distance himself from his family. SHE'S the original Uno. HE was a Wigglestein.
Instances of Father being abused in the show:
Flinching when grandpa swings his arm at him, showing he's likely been physically abused.
He still craves Grandfathers validation even after being competely dismissed by him multiple times. He wants to stand up to him but he doesn't have the strength to do so, so he slips into his old ways.
He becomes extremely depressed and self destructive when he's at first dismissed by Grandfather.
A more minor one but he clearly has trauma being forced to eat things he didn't like as a kid. It causes him to just regress completely back to the moment it happened! Kinda like PTSD.
Headcanon: monty and Ben have burn marks from grandfather. Ben has more than Monty for obvious reasons. Nigel also has his own burns from head to head battles with father. Although I do think the uno family is immune to their families fire I think they only become immune after their own powers come in.