It wouldn’t kill the SEO to your site but no, the subdomain wouldn’t likely earn any additional SEO. Typically, campaign specific pages are very light on content so they wouldn’t generally earn much SEO anyhow. You’d focus on content and building SEO for your main site, while your landing pages are your targeted work horses for a variety of campaigns.
And your best bet to ensure that this new subdomain doesn’t compete with your main site’s SEO is to hide your landing pages from search engines. (You’ll see this option in the properties pane when you’ve got your page open in the Page Builder).
You *could* feasibly create a blank Shopify page into which you embed your Unbounce landing pages with an iframe _ but _ this wouldn’t earn you much in the way of SEO. Because all Google see’s is the iframe embed code (not the content), you wouldn’t get much benefit here.