Please let Mohammad speak for Islam. After all, Mohammad is the best human that ever lived (according to Muslims)
Allah's Messenger (ﷺ) said, "I have been sent with the shortest expressions bearing the widest meanings, and I have been made victorious with terror (cast in the hearts of the enemy),... ... " [Sahih Bukhari, Book 56, Hadith 186]
The concern with Sharia Law is that allowing any foothold gives Islam fortification in the United States. Look to Europe. There are no-go zones in countries with dense Muslim populations.
The concern with Sharia is that it comes with a violent agenda that
* orders the death of all non-Muslims (Surah 2:216, 9:29-30)
* allows women to be beaten (Bukhari B7:72:715)
* Says women are inferior (Surah 2:87, Surah 4:34)
* Requires the killing of Gays (Surah 7:80-82)
When you have a religion that states these things and practices rapid expansion through the sword, migration, or persuasion (with emphasis on the first 2), you have good reason to be concerned.