Why was polyphony a failure for the Sumerians, when English functions with heteronymy?

Question

_{Source: Language at the Speed of Sight (1 ed. 2017), p. 45 Bottom.}

  Combining graphical elements to create words is an essential property of modern writing systems, but the cuneiform implementation included triumphs and disasters. Compounding and forming words out of a cue to sound and a cue to meaning were winners, carried forward into modern writing sys- tems. The disasters included representing a word such as FOOD ("gu") by the signs for HEAD ("sag") and BOWL ("sila"). The meanings of the components were relevant but not their pronunciations. Polyphony—using one sign to represent several semantically related words, all pronounced differently—was also a major fail.

1. Am I correct that the bolded is equivalent to heteronyms?

2. If yes, then how was polyphony a fail in Sumerian, by the bolded? English can't have a problem with its many heteronyms, because:

   2.1. some are startingly also Functional Morphemes like 'do' and 'quite' and

   2.2. if distinguishing the heteronym for the functional morpheme (from the other heteronyms) were too confusing and grueling, then the heteronymy'd end and the heteronyms'd be substituted by words with different spelling, pronunciation.

If Anglophones can achieve and live with this, then why couldn't the Sumerians?

Answer 1

In my opinon, English heteronymy is not the same thing. English words are not a single sign; each word is made up of multiple signs (letters) and even heteronyms have a phonetic basis for their pronunciation.

A better comparison might be Chinese languages though. Chinese characters include about 800 polyphonic characters which can make up 25% of a typical corpus by frequency. Their existence has quite obviously not been a "major fail" for Chinese characters, as evidenced by their daily use by billions of people.