Personally, I have used some of that site. I think it is very clear and made easy to use. Chord suggestions are given as in the Scaler program. I especially like making a Pianoroll melody because you can easily see the right notes to use with different scales and chords. Scaler is again very unusual friendly. That should definitely change, as we are already living in 2022.
TBH I have both Scaler and Hookpad, but I view them as completely different tools, each with their own benefits:
Hookpad is like a notebook, where you write down your sketches and ideas and keep them together for safekeeping.
Scaler is more like a tool, I use it for messing around with chords in real time, where you just throw it in a DAW and go.
tl;dr: Hookpad is a notebook, Scaler is a hammer 
I’d not seen Hookpad. I can see how it and Scaler might work in complimentary ways. If I only had to pick one I’d go with Scaler.