Sully was very unpopular because he was a favorite and was seen as selfish, obstinate, and rude.
He was hated by most Catholics because he was a Protestant, and by most Protestants because he was faithful to the king.

He amassed a large personal fortune, and his jealousy of all other ministers and favorites was extravagant.
Nevertheless, he was an excellent man of business, inexorable in punishing malversation and dishonesty on the part of others, and opposed to ruinous court expenditures that was the bane of almost all European monarchies in his day.
He was gifted with executive ability, with confidence and resolution, with fondness for work, and above all with deep devotion to his master.
He was implicitly trusted by Henry IV and proved himself the most able assistant of the king in dispelling the chaos into which the religious and civil wars had plunged France.
After Henry IV, Sully was a major driving force behind the happy transformation in France between 1598 and 1610, in which agriculture and commerce benefitted, and peace and internal order were reestablished.

After the death of Henry IV Sully published, in the deceased king's name, his ‘Grand Design’, a plan to stop the religious wars.
His starting point was that the three churches (Catholic, Lutheran and Calvinist) were there to stay.
He planned an international organization, consisting of a Europe of 15 more or less equally strong powers, incidentally dissolving the Habsburg empire and thus making France Europe’s strongest state.
A balance of power mechanism and a permanent assembly of ambassadors should prevent wars in Europe.
Military power would only be needed towards Russia and the Ottoman Empire.
Sully was very unpopular because he was a favorite and was seen as selfish, obstinate, and rude. He was hated by most Catholics because he was a Protestant, and by most Protestants because he was faithful to the king. 
He amassed a large personal fortune, and his jealousy of all other ministers and favorites was extravagant. Nevertheless, he was an excellent man of business, inexorable in punishing malversation and dishonesty on the part of others, and opposed to ruinous court expenditures that was the bane of almost all European monarchies in his day. He was gifted with executive ability, with confidence and resolution, with fondness for work, and above all with deep devotion to his master. He was implicitly trusted by Henry IV and proved himself the most able assistant of the king in dispelling the chaos into which the religious and civil wars had plunged France. After Henry IV, Sully was a major driving force behind the happy transformation in France between 1598 and 1610, in which agriculture and commerce benefitted, and peace and internal order were reestablished.
After the death of Henry IV Sully published, in the deceased king's name, his ‘Grand Design’, a plan to stop the religious wars. His starting point was that the three churches (Catholic, Lutheran and Calvinist) were there to stay. He planned an international organization, consisting of a Europe of 15 more or less equally strong powers, incidentally dissolving the Habsburg empire and thus making France Europe’s strongest state. A balance of power mechanism and a permanent assembly of ambassadors should prevent wars in Europe. Military power would only be needed towards Russia and the Ottoman Empire. 
Maximilien de Béthune, Duke of Sully