['Ali 'Imran 18]
شهد الله أنه لا إله إلا هو والملائكة وأولو العلم قائما بالقسط لا إله إلا هو العزيز الحكيم
Allah witnesses that there is no deity except Him, and the angels and those of knowledge - maintaining in justice. There is no deity except Him, the Exalted in Might, the Wise.
Allah is Al-Muqsit, "The Fair, The Just". It obviously follows that Allah's Law is also fair and just.
People are fallible.
Most of what people call "Islamic Law" is just human attempts at interpreting and applying Allah's Law. Without divine infallibility there is no guarantee that it was interpreted perfectly or that a judge makes no mistakes in their judgment, so attributing any unfairness to Allah or Islam is presumptive.
That said, there is also no guarantee of "equality" between men and women. Men and women are not the same, gender roles are well-established in Islamic jurisprudence, and Islamic law explicitly accounts for this. There will always be laws and rules which apply to men but not women, or vice-versa.
There is also no guarantee that everything will be fair in this world; any worldly injustices will be reconciled on the Day of Judgement. This includes people who escape worldly justice as much as any who are unjustly punished: All will be known and accounted for.