Top 30 SEO Rated by Neil Patel as one of the top 30 SEO's in the world in 2017 and consults on some of the most complex SEO problems for companies internationally including but not limited to; Bold, Porche, D2L, Ceridian, Porter Airlines, Neil Patel and more. CEO and Co-Founder of NAV43 (a Technical Search Agency) in Toronto as awell an SEO software company. Published on Forbes.
TL;DR -> Yes you will risk it if you don't perform a proper audit and migration from the original platform.
Any type of architecture changes can 100% ruin your SEO if you are not migrating content and topics correctly. Many people assume this is limited to URI structure but underestimate the power of Topical hierarchy and the content, internal linking and URI structure which play a crucial role into any migration effort.
If you have any questions about the migration process, give me a shout.
Decent free migration checklist: https://searchengineland.com/site-migration-seo-checklist-dont-lose-traffic-286880
Great Question! As I'm sure many other SEO's here have mentioned.. you should absolutely be leveraging the sub-folder as opposed to the sub-domain as Google and Bing associate sub domains as separate entities with their own value metrics outside of the primary root.
The only real time I would recommend a sub-domain over a sub-directory would be due to platform restrictions (Shopify, Wix, etc) where you don't have access to build an infrastructure to support nested content from multiple hosts.
Definitely worth chatting about, however you shouldn't have any problems using the sub folder with any other host you'd like.