The first season of Amazon is all about two Kamen Riders who were created by human experiments and become part of a kaijin hunting group. Honestly pretty gritty and dark stuff. And then of course the focus on the bromance was a plus for me at least.
Both shows have Yasuko Kobayashi on the writing staff (she was a sub-writer on GoGoFive and wrote every single episode of both seasons of Amazons), Masashi Taniguchi among the main cast (Go Blue in GoGoFive and Amazon Alpha in Amazons), and put a focus on family dynamics.
Both are not kid friendly tokusatsu shows, with Amazons going more into morally grey and some gruesome visuals, while Garo has some monsters with creepy undertones and some guest characters will show an occasional boob.