Your position on SERPs is defined by the search engine's understanding of what you are and what's your quality.
Interlinking, neighbourhood, domain ownership and what else can be used to refine that understanding.
Do the subdomains with diverse topics haze the understanding about the main domain? Do the subdomains dye each other?
Google says it handles subdomains and the main domain as different domains. The example.com, www.example.com and my.example.com are different domains.
Someone has mentioned that in Google Webmaster Tools, under the tab "What Googlebot sees" for his main site profile, he sees keywords that are mentioned on his subdomains but not on the main site.
Checking another family of websites on the keywords in GWT did not reveal any leaking, even as the root domain and subdomains were cross-linked.
The SEO community suggests opposing to each other theories and opinions. That means some assertions you find on the web are true, some false, and that means - no-one knows for sure.
As far the question was raised, there must be a concern. To stay on secure side and keep things simple, go for separate root domains.
Considering subdomains versus subfolders is easy if you know who you are, where is your focus, how you want SEs understand you.
For example, if you cover two areas: [web development] and [search engine optimisation] and want to be highly ranked on each of them, and these are quite detached operations for you, then keep each on a separate (sub)domain. Small leakage between subdomains will possibly not to cause any big harm in this case.
However, if you position as a [web agency] providing a holistic approach to SEO and development, then you might want to appear and be ranked as a medley of both. In this case subfolders work better for you.
Subfolders however would work better if you position yourself as a [web agency] providing a holistic approach to SEO and development, and you want to appear and be ranked as a medley of both.
In short, there is no one-to-one answer; it all depends on the context.
Vasilis Vryniotis analyses the question in his article Multiple Domains vs Subdomains vs Folders in SEO for Web SEO Analytics, 27 Sep 2010.
Root Domains, Subdomains vs. Subfolders by Rand Fishkin to Technical SEO, 29 January 2009.
Subdomains compared versus root domains and subfolders in SEO point of view.