I don't think of Shetlands as particularly tiny. They seem to eat as much as a larger sheep anyway.
If the sheep would be as lawn mowers, why not get wethers? If you have breeding ewes, then you have the problem of keeping or borrowing a tup (from where, in a city) and of disposing of the lambs - which when you just have a couple is difficult to do. If they would be going for meat, where is your nearest slaughter house?
For overwintering your sheep, they will need hay, and they may need some in summer too on such a small area, so will need a supplier and storage.
You will need access to a large animal vet, which can be difficult in a city. Many large animal vets will share medicines amongst a number of small breeders, but in a city you'll be on your own - many products come only in large sizes, for average commercial flocks.
You will need to learn to shear, or with Shetlands, to roo - not difficult and quite enjoyable, but if you don't do that you're unlikely to find a shearer in town.
On the plus side, you would have enough fleece each year to support a reasonable amount of spinning
and Shetland wool is delicious to spin, mostly
Another plus is that livestock diseases would be uncommon in a city, unless you live beside a zoo.
Just a few points I can think of, but hopefully others will have more. Maybe a bit more info on your precise wants and position would help us to think of the practicalities. For example, when you say 'a city', how big is that city and are you in the centre or on the outskirts?