What are drawbacks of installing outdoor sediment filters this way?
This is a pretty common setup to get high flow from smaller, cheaper filters. Two 10 gpm filters can be used to effectively get 20 gpm overall flow.

In this particular install the flex pipes, mix of galvanized and brass parts, and lack of shut-off valves and pressure gauges screams amateur installation to me, but I don't see anything that prevents it from working.