My personal opinion in this matter ok?

Some threadless adapters are just cheap as hell, the only descent is the nitto, i had the longer one (nitto has 2 models) and worked just fine.

Advantages?? well is easier to swap stems if you are having problems in the fitting, is a PITA to install and remove quill stems, just not as fast as a threadless stem. Besides that I dont know if its stiffer than a quill stem, I have used both in the same bike and cant tell the difference.

Disadvantages, unless u get a 1inch to 1 inch adapter the thing will look baaaaaaaaaaaaaaaaaaadddd... The bikes simply look bad IMO. The other problem is that source 1 inch threadless clamp with oversize handlebars is like hard, the other thing is that a good and nice 1 in threadless stem will cost way more than a 1 1/8 one. Cinelli has some 1 inch threadless stems that are simply fantastic but will cost more than a carbon one in 1 1/8.

Good luck