Hi Luc,
Yes partly because they can breed in sweetwater and the other unknown is how they will interact with local species. There's been a truckload of money, time & effort spent on local restocking and of course on Mary River Cod recovery, sooties might not "play well" in the sandpit with others and start killing them.
Lastly there's always a threat of parasites and disease being spread when fish get translocated.
Sooties have been released into the Mary River and are surviving so far, but I'm not aware of any breeding as yet; most caught are larger fish. The river substrate would be adequate for sooty grunter to breed in most SEQ waterways, but the lower water temps may be the savior.
Here's hoping they can't handle the cold........
Cheers,
fitz