I somehow came across the app of observation.org, yesterday.
And asked me a similar question, it looked as if there would be challenges like in SEEK, and I thought, maybe I I could get some new inspiration and use it a bit/look at the challenges.
But, after doing an account the only challenge that I could see, was finished since months.
And I decided, that it really is better to concentrate on Inat, as I love it. :)
So I deleted that app of observation.org.
Just do what feels right for yourself. :)