It’s not that “tricky” – likely because back when my daughter and I were selling checks as a “merchant” and had affiliates sending us customers we had about 40 sites going. The SEO can more easily be tailored to each niche and (may) improve stickiness. For example, someone looking for photographs of the Indian River Bridge will find only those images there – the Fort McHenry images are on another site. However, once they get to my main gallery (via a link) they may get distracted by other collections. But who knows – who really knows, eh?