Make sure that the treats/chews are actually safe.A lot of commonly recommended treats(for example Purina,Pedigree,and even some high end brands)can be dangerous or at the very least incredibly unhealthy
There are tons of ingredient loopholes that not many people know of,for example E&C approved antioxidants/preservatives which actually allows for BHA/BHT/ethoxyquin to be added(as it definitely is in the US,that I can guarantee!).Another thing that could get ignored would be digest,which is another term for biproducts.Not all biproducts are bad,but they're not created equally and when put under blanket terms there is no legal definition to prove that they're safe or not or from good sources(for example,roadkill or slaughterhouse waste is not uncommon with such unspecified ingredients).I also see animal fat a lot with various brands,and this is once again unspecified so you have no clue if it is safely sourced or not so the danger of unsafe sourcing is once again present
I know that Greenies also have gelatin(boiled bones)in them which isn't good and they're quite well known for causing serious digestive upset in dogs,which isn't that surprising with the ingredients but it is still a risk for rats and other small creatures.