No I do not agree to that blanket cover - Marriage between Shia and Sunni is Halal - though both sects discourage their women to do so - control over the beliefs of next generation 
On personal note - whenever I am at my Iraqi (Shia) friend’s home - I have joined in prayers with them with and their granddad leading the prayers ( you know I can’t practice taqqiyah :-)) I know my prayers are valid though I feel the old sometimes feels a bit uncomfortable with me in the group. (My friends & me always laugh about it - the discomfort of the old man)