RSI contract problems

Hi there, I'm Gabriel.
I've been offered the infamous RSI contract by an employer. It would basically mean about 300 Euros more on my net income. I'm a EU citizen. I'm visiting with my sister in BXL and I'm not registered with the commune. I'm planning to stay indefinitely, I'm married and I have kids.

There is little official info to be found and I find it difficult to just trust in my employer fully, he might plainly be wrong about something.

I think there are a lot of people with personal experience on the matter on the forum. I would like to asked them, but also any legal experts if there are any on the forum, what are the eventual downsides and limitations of this "expat" contract. How did this impact your life and relocation here.
The term non-resident confuses me a bit. Is it just a tax thing or will I actually not be able to get a Belgian ID? Or a citizenship in the long term.

Thank you very much!
It's so good to have found this community.

I will like to buy an appartment or a house in the next year maybe. Also, with kids I need toake sure public schools are available to me. And the monthly child "allocation".