SGML Parser Recommendations

I have heard advice (opinions) suggesting that the FrameMaker 13 (2015) Parse Structured Document functionality is dated and not always reliable. Does anyone have experience with a trusted SGML parser that supports Unicode, for checking markup before delivering it to my customers? I understand nothing is 100%. I'm just looking for comments about good experiences. I am saving out of Structured FrameMaker as XML, with post-processing that produces DTD-compliant SGML.

Thanks for your interest and time,

Mongo-Man