It is a moot matter to criticise Shi'a translators for their one-sidedness when, as far as one can see from the internet, no translation of Nahjul Balagha in any European language exists from Sunni scholars, although both Sunni and Shi'a recognise the legitimacy of Ali ibn Abi Talib, may G'd be pleased with him, and the excellence of his discourse.
If Sunni muslim scholars differ with the views expressed in the comments, may they publish a better translation with better commentary, striving with their Shi'a brethren in good work!
Concerning the underlying source, one would have to attribute an indefinite number of stars. The 3 star qualification results from the deficient qualities of the book as such. Except from the text quoted from Hadrat Ali, the typeface is simply too small or even much too small for anyone other than those with excellent vision. For those readers over 45 or so, not even reading glasses suffice, a magnifying glass is required to read the smallest type.
Nearly every page contains spelling and / or syntax errors in English. Scanning the text and running it through syntax and spell checking programmes is a must.
Some doubts may be entertained concerning the inner qualifications of the translators. For example, Hadrat Ali is translated as having said that G'd does not come from "non-existence". What Arabic word is "non-existence" supposed to translate? Ex-istence comes from Latin ex-stare, meaning outside or out of being. The opposite would be "in-istence". In this and similar cases, the translators would be well advised to put the transliterated Arabic word in brackets after the English, as one often sees in scholarly works. This lowers the risk of misunderstanding.
Lastly, the translators inadequately convey the eloquence of Hadrat Ali's Arabic into their translation, although this is admittedly difficult. To compensate, they might add an introductory chapter explaining and illustrating why Hadrat Ali's use of Arabic is considered, after the Noble Qur"an, to be the epitome of eloquence.