It is inevitable that you will see some rankings drop, no matter how well you check all the "migration best practice" boxes. I've never seen a migration done that resulted in no, or even positive changes right after launch. However, you can greatly reduce the time that your rankings fluctuate and stabilize again by taking all the proper steps. There are simply to many to list, but the main issues are typically around URL changes. Ideally you'l want to make sure there is a 301 redirect in place for every page that has a URL change. Also, make sure to submit a new sitemap via Google Search Console the second you launch the new site.

The thing to keep in mind is that any time Google sees big changes, it will "step back" a bit from the site to reindex and reanalyze everything. Whether this is a platform migration, an extended server outage, or a domain change, it almost always results in a temporary decrease in ranking. Whether or not (and how soon) you come back stronger will depend on how well you handled the migration and how much better the new site is from an SEO perspective. Shopify is a decent platform and like any of the major systems, can be optimized to rank quite well in most industries. I'll leave you with this decent resource - https://moz.com/blog/web-site-migration-guide-tips-for-seos (I'm not affiliated with Moz in any way).

This really depends on the amount and type of products you are selling.

From an SEO perspective, Shopify is great for small sites (let's say, under 100 products that don't frequently change).

However, if you have a business with lots of products and that change regularly, it might be a bit riskier. This is because Shopify lacks some of the page indexation/management functionality of some other platforms.

Take a read of these, as I think they will help:

http://www.codeinwp.com/blog/shopify-vs-magento-vs-woocommerce-vs-bigcommerce-vs-wix/
http://tripleseo.com/shopify-seo/
https://ignitevisibility.com/shopify-seo-review/

No,Shopify won't cause any SEO issues, becuase like other systems e.g wordpress , magento etc , it's themes are made in HTML, css3 and js and it's own templating language called Liquid. As you can see, you can optimize your site for any SEO concerns. I am also a Shopify Developer and i am telliing you that shopify store can be easily optimized for SEO

I'll take a different approach than the other answers: yes, it's likely you'll have issues switching to Shopify if you don't plan absolutely everything.

Now, if your site is making no money or a limited amount of money, the stakes aren't that high. But if you're making a substantial amount of money for your business from your ecommerce platform, then the stakes are absolutely high.

Here are the questions to ask:
-Do you have complicated, custom products that require configuration? If so, then really evaluate if the switch makes sense and can be done within scope.
-Do you have tons of product, over 1,000? If so, then take a good look at whether Shopify is the best.
-Are you using features on your legacy platform that are crucial and not built into Shopify?
-Are there certain traits on your site such as internal links or category page content that is difficult or different to implement on Shopify?
-The list goes on

Don't just flip the switch and hope it works. Shopify can be great for SEO, and better than your current platform. Or it can be way worse, depending on where you're coming from.

I don't care if you go to this link or not, just wanted to let you know I've thought about this a good amount and I'm continuously updating this page on all Shopify SEO topics - https://www.greenflagdigital.com/shopify-seo/

Shopify does have SEO issues these are as follows:
1. Inability to edit tag pages
2. Duplicate content
3. Duplicate page titles
4. Forced URL structure
5. Locked robots.txt
6. No customization of pages
7. Limits on page titles and meta descriptions
8. Poor initial template selection
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath