The closest masjid to my residence is Shia. I have been going there for convenience even though I was raised as a Hanafi Sunni.
Overall, as the congregational salah is not that much different than the Sunni salah. They don't fold their hands during standing, and slight differences in dua. However, they supplicate the adhan and ikamah with something that sounds like 'Ali is the Wali of Allah' after the standard 'Muhammed Al Rasul ulah'. I understand that this means Ali is the governor or administrator of Allah.
Would Sunni Islam have an issue with this as a Bidaa? Would it risk the acceptance of the Salah?