All corrections
1
Claim
The Middle English merger of the vowels with the spellings ⟨our⟩ and ⟨ower⟩ affects all modern varieties of English and causes words like sour and hour, which originally had one syllable, to have two syllables and so to rhyme with power.
Correction

This is too absolute. Modern descriptions of English pronunciation do not treat hour/sour as uniformly two-syllable in all varieties; syllabification varies by accent and even by speaker.

Full reasoning

The sentence makes two categorical claims: (1) that this development "affects all modern varieties of English" and (2) that it makes words like hour and sour "have two syllables." Authoritative phonetics sources do not support those absolutes.

Peter Roach's English Phonetics and Phonology says that in BBC pronunciation (RP), words such as hour are "probably felt by most English speakers ... to consist of only one syllable." That directly contradicts the statement that the change makes such words two-syllable in modern English.

A linguistics textbook also notes that words like hour/power have "murky behaviour": some speakers judge them as one syllable, some as two, and speakers are often inconsistent. That means the article's universal wording ("affects all modern varieties" / "to have two syllables") is not accurate.

A safer formulation would say that these words are often realized in ways that rhyme with power and that their syllabification is variable across dialects and speakers, rather than uniformly two-syllable in all modern English varieties.

2 sources
Model: OPENAI_GPT_5 Prompt: v1.16.0