As far as I know, the Dutch government has been pretty slow in their EIDAS implementation, even (nearly?) missing some deadlines, maybe the Belgian government has a similar problem?
For some integrations with Wallonia, we are currently forced to use the federal integration, since their implementation hasn't even started yet :(
You are correct about eidas, it seems to be handled by the federal government currently, but there's not much documentation on it ( Belgium) and i have seen some applications integrating that way. So i suppose the issue is documentation and discoverability.
And the partial conflict of interest with itsme
Like is the case with my own government, I don't have very great expectations of Belgium's federal government and their IT projects. I'd wish countries could just share their implementations with other governments. Hell, I believe any code developed with public money should be open source for the benefit of everyone, but governments aren't a fan of that because companies charge more for developing software in the open.
This was quite a frustrating experience. Some time after leaving the country I got e-mails saying that SKAT, the local tax authority, had documents/messages for me to read on the secure message platform eBoks.
However, try as I might, I was unable to convince anyone to restore access since I was no longer a resident. I agonized quite a bit over this since I dreaded inadvertently being a fraud.
It would have saved me a lot of grief if the Danish e-gov services would have federated with Belgian and other EU identity services.
Edit: Forgot to say this is something The Netherlands already does pretty well. Dutch e-gov services, or at least those from Belastingdienst federate with many European idenity service providers including Belgium. Great for cross border workers.