This has already been outlined quite a lot on the thread, but just to name few:
The number one issue is that governance structures are complicated and work together in synergies. If you add a whole new element, the idea that you would somehow leave it to later, to even talk about how it would be consituted, is just incredibly foolish.
There is a strong tendency do this in legislation in recent years, and people make this argument that it is just about "recognition" or support and it will all come out in the wash because after all we all want things to work out right. But it very often ends up with bad policy that has bad effects. If the people proposing this weren't prepared to do the work to show what kind of body they were proposing, with some significant detail, there is no reason anyone should have trusted them to do the work later.
There is nothing to say, either, that once this was constitutionalized, it would be accomplished through legislation, rather than through some other means.
In particular, it would have been important to define what is meant by "advisory" body. This could mean what is in the end a totally ineffective waste of money and air, all the way through to a body that has some kind of ability to propose policy, to some kind of veto power, who knows?
The other element that is going to be very concerning to many is the institutionalization of ethnicity as defining how much, or what kind of influence, people have on government policy. That's a pretty direct over-turning of the principles western democracies have worked under. The same principles, by the way, that said that actually, it was illegitimate to restrict voting by racial group.
And to take that thought a little further, it's not only directly concerning that it might mean you now have some citizens with more avenues than others, it's concerning because you have set a precedent that political rights, at the constitutional level, can be tied to ethnicity. Constitutions are long term documents, meant to last hundreds of years. Where could a precedent like that take people in 200 years? What if the public mood changed, could other group's right be defined based on things like ethnicity, or how long their ancestors had been there?
With those kinds of considerations at stake, it's pretty natural that in order to vote yes, people would want some clear information about what they were voting for.
You have to wonder on what basis people think it would convince anyone to say, "aw, no, we don't need to have any clear outline of what this would look like or how it would function." If they believe it's a good idea with substance they should be keen to talk about those things.