Google recommends responsive design
"Why responsive design
We recommend using responsive web design because it:
Makes it easier for users to share and link to your content with a single URL.
Helps Google’s algorithms accurately assign indexing properties to the page rather than needing to signal the existence of corresponding desktop/mobile pages.
Requires less engineering time to maintain multiple pages for the same content.
Reduces the possibility of the common mistakes that affect mobile sites.
Requires no redirection for users to have a device-optimized view, which reduces load time. Also, user agent-based redirection is error-prone and can degrade your site’s user experience (see Pitfalls when detecting user agents" section for details).
Saves resources when Googlebot crawls your site. For responsive web design pages, a single Googlebot user agent only needs to crawl your page once, rather than crawling multiple times with different Googlebot user agents to retrieve all versions of the content. This improvement in crawling efficiency can indirectly help Google index more of your site’s content and keep it appropriately fresh."
Some things may want to change though are, showing links to download apps for the platform detected the user is on.
Showing links for maps etc in the format that work for the target platform device, etc.
You may have reasons not to use responsive design, e.g. Legacy etc.
But would have thought a Greenfield project would be responsive by default, requirements depending.