Well, it's slower because of a join that exists in SQL (the relationship between your 'field/value' table and the entry. Apart from that, as you said indexes are similar.
The fun thing about NoSQL skeptics is how they think of only the current scenarios they work with, and they won't believe you until they get burned by it. So be it.
But why would I waste developer time doing that if I can only do db.table.insert(obj) - in MongoDB for example (obj is a JS object)
Also, finding all objects with a field named 'field1' and value '3' is slower if you do that in a relational DB (and that's the simplest case)