Everything was good in that part of Shankaracharya "Prapanchasara-Tantra" which is dedicated to Vaishnava-Sadhana. But only in a few introductory Verses which tell that Shree Ashtakshara-Narayana-Mantra is the Highest One among all the Vaishnava-Mantras (yes, that is more than true according to Shree Ramanuja-Siddhanta).
Crap happens further, when Shankara starts to interpret Shree Ashtakshara-Narayana-Mantra via "soham", "neti-neti" and sheer adwaitic nonsense, which sounds like rubbish from Orthodox Vaishnava point of view.
Actually, Shankaracharya mayavadic tikas are definitely the worst tikas in all the Agamic/Tantrik history!!!!!!
Look (the cite from Hindi edition of "Prapanchasara-Tantra"):
