This is an interesting and puzzling phenomenon I started exploring some time a go and still have not yet reached a conclusion, so I'll give more of an impression here than solid evidence.
The rule in fact is that with a single verb without nominal subject (noun, pronoun, citation, or relative clause) the predicative adjective should become an adverb:
Dancado estas bela ←→ Danci estas bele
La nokto estas malvarma ←→ Estas malvarme
Subjectless verbs like in the second example do not have a clear reference, they just name a state/action.
In case of an ellipsis which refers to a noun named just before, however, there should be no problem to stay with the adjective:
A: Kaj la vetero?
B: [La vetero/Ĝi/Tiu] Estos certe bona!
A problem rises when the reference is not a clear object, but rather a whole sentence or a situation. Traditionally tio or ĝi was used as a subject in such cases:
Tro multe da homoj mortas pro malpurigo de la trinkakvo. Ĝi/Tio estas vere problema.
It seems to me that there is a tendency in modern speech to omit the pronoun in such cases, switching to the adverbial form instead of the adjectival one - although this is not how subjectless verbs were intended.
An explanation for this may be that in early Esperanto ĝi was used, as in English, to express an empty subject with atypical subjects (Ĝi estis varmega posttagmezo "It was a hot afternoon"). This has (rightly) totally dropped out of use, perhaps taking some of the justified uses with it.