Were all the american grievances really justified, or were the british actually being more reasonable than most americans have traditionally believed?

Personally, i do believed that the British was among the most reasonable empire that ever created colonies. Because they treated their colonies as business partners rather than country-slaves.

If you pay attention, ex-british colonies tend to be way more advanced compared to colonies from other countries

For example, United States which dominated north America, Singapore and Malaysia which dominated South East Asia, India which dominated South Asia, South Africa which dominated the whole African Region, etc.