Microsites: Lamborghini with one gallon in tank
A few years ago, microsites were one of the hottest ammunition deployed by brands to promote a new product launch, new campaign, generating new targeted leads. This allowed companies/brands to curate segmented information about the topic in question and also convinced the organization that it will generate more impression among customers as it is lightweight and focused on one topic. Unfortunately, following this approach could lead to negative results and can decrease organic traffic to your brand website.
Brands creating multiple microsites with hope or with an idea that backlinks to the main website will trick search engines to rank the website higher. This is like you know one thing on the test and you are trying to cheat to get a higher score. But as a student, you forget professor (GOOGLE) is not going to take a nap and let you do whatever you want.
Professor(GOOGLE) have a large number of tools (Google Search Algorithm) to catch you if you are cheating and you are possibly affecting your reputation for the following test also (Link Dilution). Microsite also confuses search engines by creating duplicate content. Microsite uses the content or part of the content on the main website. Duplicating the content will confuse google to either filter out either microsite or the website for the search result. This is not a good strategy for organic traffic to the website as most of the time will show the result you don't it to appear.
Google track different analytics ranging from the user metrics on the websites to social media engagements. Having microsite is like having another website which will eat a portion of the metrics and can decrease your rating in that department, affecting organic traffic for long term success.
There are strong arguments by brands competing in the one-mile race and it might be hard to counter the argument. For example, if you are running TV campaigns, Microsite can be a boon and can help you get excellent results.