I'm not a developer so excuse me if my question sounds somehow wrong. The client wants to us Rest web service to create/modify these objects in Netsuite: customer, vendor, item, invoice, bill, inventory transfer, customer and bill payments. Is it possible? Because I'm reading various sources that people struggle to do that and that actually Rest functionality is in Beta stage actually and some objects are accessible only upon request to participate in Beta program.

Apparently REST will be GA in 2021.2 (roughly Oct 21), but all release notes are subject to change I think.

but in the realease notes 2021.2 I can find that Rest Datasets are out of Beta, nothing else.

Take a look at RESTlets as an alternative, that's the route I'd take (at the moment anyway).

and what's wrong with the SOAP which has everything and its not beta? Because its to complex too handle it?

REST is newer, lightweight (aka faster), and in most cases more flexible when compared to SOAP.

nobody likes generating xml by hand, especially not with all the namespaces and abstract classes suitetalk uses