Thank you for your comment fortunejack!

Nice of you to note that I am wearing their signature.

So you mean that the Nexo and celsius can also be scammers, but so far, the way they seem is legit, right?
There is a difference between what is legit and what is a scam. A project may be both or none or either of the two.
My question is that what could be the "most significant factors" that people evaluate those services seem legit or scam?
There is no such yardstick.
For example, since harubank seem to provide the highest interest among other defi & crypto finance services, so I deposit a small amount of BTC to test the service (done KYC & withdraw) and worked really fine.
For me to gain trust is to try the service to see if they are keeping the promise. Of course, I wouldn't deposit a substantial amount until I have more than 80% trust, but my way is to experience the service.
The above two statements explain clearly how scammers attempt to gain confidence of newbies and gain trust. MLMs are built in a way that the new investor's money is used to pay the old investor. Gradually this cycle will stop and the site will exit scam. Its not possible to predict which one is scam but the signs are there.
So, I'm curious to know about how and why people are so sure and stigmatize that the new and not-well-known service is definitely a scam.
Read the post made by o_e_l_e_o, if you still believe it is not a scam then I cant help you clear your confusion.