However, there are quite a few Sardinian records on the LDS site. Some records have been indexed, but not all of them.
To see what they have, you need to search the catalogue:
https://www.familysearch.org/search/catalog
entering the place name where it says "Search by place". It's not enough to put Sardinia or Sardegna: you need the name of the town or province.
For example, if you're looking for a place in the province of Nuoro, type "Nuoro", then click on the result that says "Italy, Sardinia, Nuoro".
You then need to look for a result that includes the words "Registri dello stato civile" (Civil Registration registers, ie. registers of births, marriages and deaths).
When you click on that, you may get a result like this: "Records of Italy, Nuoro, Nuoro, Civil Registration (Tribunale) are available online, click here."
That means they've been indexed, so you can search for the name of the person you're interested in.
If they haven't been indexed, you have to find the town or village you want, then the microfilm that contains births (Nati), marriages (Matrimoni) or deaths (Morti), and then look for the year you're interested in. The records are obviously in Italian.
Some registers have an alphabetical index of names at the beginning or end of the year, which makes it quicker to search, but it's still a lengthy process if you don't know the year you want.