I'm with Hunky Bill on this one.
We've done dozens of migrations now over different platforms for some quite big brands and URL structure doesn't matter in itself one jot in SEO.
What does matter (and coming back to the original question) is trying to match URL structure during a migration to Shopify and doing 301 redirects correctly.
Most people mess that up and hence run into SEO issues on Shopify and blame the platform.
A 301 redirect to exactly the same content (e.g. from /blah-blah to /something-weird/blah-blah) - would not cause an SEO position drop (we've tested this with our own keywords which rank nicely on page one). Assuming markup, handle, content and links stays the same (those things do matter).
What we've done in some cases is some pre-prepping of the content to be migrated - so that it fits with Shopify's URL structure (which isn't going to change).
e.g. if old content is on WP - you can slowly and easily start to change existing content URL structure on WP to match what it will be on Shopify and monitor positions. That way when you make the move - nothing changes with URL structure (which does help in narrowing down any problems).
We are also facing same problem with Shopify theme regarding to remove "/pages" from some simple pages as shown below.. We need solution on it.
How to Remove "/pages" from below Shopify pages URL:
Any help appreciated.. thanks.