I have an ecommerce site with different store views for different regions. In the UK there is a 'en-gb' locale version priced in GBP, in the US there is a 'en-us' version priced in USD and also a 'en' version for customers elsewhere that do live in the UK and US (the 'en' store is in Euros).
For each page there is a block of hreflang tags in the header with the different store views listed.
The SEO (Snake Oil Salesmen) company believe that I have to endure believe this is wrong and that the search engines will crawl the generic 'en' version rather than the 'en-us' version that would apply to the US (or the 'en-gb' version that would apply to the UK).
There is no reason evidence provided by the SEO marketering people to back up their claim and a brief Google search did not give any results.