Salaam
There is something that I have noticed, that not too many people have addressed. It is the problem of Sunnis, particularly those who are younger and live in Western Countries, being influenced by the ideas of Shias. For example, I have encountered a Sunni individual who expressed the opinion that the problems with Islam started after the Prophet (PBUH) passed away and the Sahaba started quarreling. This individual then went on to criticize the Sahaba (May God be pleased with them). I didn't realize at the time, but it is clear that this person was influenced by the ideas of Shias and without a real understanding of history and reliance on accurate Hadiths, the uninformed Sunni then accepts these ideas. Even if the person does not convert to Shiism, they become influenced by it.
Furthermore, many Sunni institutions/Masajid/leaders promote the idea of unity between Sunni and Shia and are afraid to be labelled as Sectarian. So many of these Sunni youth don't find responses to Shia accusations from their Masjid.
Many are also plainly ignorant of the beliefs of the Shia. I see this as a problem especially when we have many Modernist or Liberal Muslims who don't look to authentic sources and come up with their own theories on Islam. They find the Shia narrative attractive and promote it, or at least give it credence as an alternative to traditional Islam.