It will appear in search results separately from your main domain

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If you place a landing page on a subdomain, it will be displayed in search results as a domain separate from the main domain. Since Google does not display more than a certain number of pages from the same domain, if there are multiple landing pages on subdomains, they will be displayed multiple times, which can lead to an increase in organic search traffic.

Likely to be considered an independent service
In the case of a subdirectory, the product name or other name is added after the main domain, whereas a subdomain is displayed before the domain of the main site. This makes it easier for users to see and increases the chances that it will be recognized as an independent service.

Disadvantages of placing on a subdomain
On the other hand, the disadvantages are as follows:

SEO evaluation is scattered
Since a subdomain is recognized as a separate site, even if the main site has a high SEO rating, the landing page cannot benefit from it. The SEO measures of the main site will also not be effective, so it is not suitable for aiming for a high ranking in preliminary searches.

Difficult to manage and operate using analysis tools
Subdomain landing pages are treated as separate domains from the main domain in tools such as Google Analytics and Google AdWords, making management and advertising complicated. Also, if you are using a CMS such as WordPress to operate subdomain landing pages, you will need to manage them separately from the main domain.