Assalaamu alaykum. Is Ash-Shamaa'il Al-Muhammadiyyah completely authentic?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

If you mean Ash-Shamaa'il Al-Muhammadiyyah by Abu ʻEesa At-Tirmithi then it is a great book in its field; however, there are some weak, inauthentic ahaadeeth cited therein. Shaykh Muhammad Naasir Ad-Deen Al-Albaani classified the ahaadeeth cited in this book and summarized it in his book Mukhtasar Ash-Shamaa'il Al-Muhamadiyyah, distinguishing the authentic ahaadeeth therein from others.

You should know that no book is free of criticism save the Book of Allaah; all other books consist of human production. Ash-Shaafiʻi said, "Allaah, The Exalted, willed that no book would be infallible save His Book."

Allaah knows best.