From Elsevier databases Scopus has integrated thesauri like GEOBASE Subject Index (geology, geography, earth and environmental science), EMTREE (life sciences, health), MeSH (life sciences, health), FLX terms and WTA terms (fluid sciences, textile sciences), Regional Index (geology, geography, earth and environmental science), Species Index (biology, life sciences), Ei thesaurus (controlled and uncontrolled terms) (engineering, technology, physical sciences). As you see, the last one includes uncontrolled terms. Scopus also integrate author keywords which are uncontrolled keywords supplied by the author of the article.

When searching the field Keywords in Scopus you won’t get just controlled vocabularies, you will also get uncontrolled vocabulary from Ei and author keywords.

This reference from PubMed:
Nicolau B, Marcenes W, Bartley M, Sheiham A.
Associations between socio-economic circumstances at two stages of life and
adolescents' oral health status.
J Public Health Dent. 2005 Winter;65(1):14-20.

It does not have the MeSH terms (from PubMed) or EMTREE (from Embase) integrated in the Scopus reference. Just author keywords as you see at this screen shot:

Another example of a Scopus record with no EMTREE terms. (No MeSH headings exist yet because it's a PubMed in process record).

And the same reference in Embase with EMTREE terms:

The following reference is from from PubMed:

Anderson C.
Breast cancer. How not to publicize a misconduct finding.
Science. 1994 Mar 25;263(5154):1679.

See the MeSH-terms at the screen shot. The terms with * -sign means it's a major MeSH heading:

Not all major MeSH headings are included in Scopus reference:

Ei thesaursus, sometimes called Compendex thesaurus, is not properly implemented either. On this screen shot you find a record from Scopus with no Ei thesaurs terms attached:

And here's a screen shot from the database Compendex showing the same record with Ei thesaurus terms attached:

When testing and comparing Compendex with Scopus, quite a lot records didn't integrate Ei thesaurus, but when it exists on Scopus records it has integrated both main heading, controlled and uncontrolled terms properly.

Unfortunately, subject search in Scopus has a lot of disadvantages:

  1. It’s impossible to browse the keywords and thesauri integrated in Scopus.
  2. The thesauri are inconsequently integrated, sometimes no MeSH terms, sometimes no Emtree, sometimes not all major MeSH headings.
  3. Uncontrolled terms are mixed with controlled ones and not possible to separate when refining a search.
  4. You can’t choose which thesauri to use.
  5. No mapping of terms as in Embase and PubMed.
  6. No possibility to explode terms.
  7. No integration of MeSH subheadings
    Conclusion: This means Scopus is impossible to use for refined and comprehensive subject search. That means you have to use PubMed to properly use MeSH terms, Embase to properly use Emtree and Compendex to properly use Ei terms. Of course Scopus is not built to substitute the Elsevier databases. That’s why I don’t think Scopus will ever implement the thesauri of the Elsevier databases properly. But why subject search of MeSH terms is not properly implemented when PubMed is a free source is very strange.