In my work as a consultant, I often meet screenwriters who are hoping to get into the marketplace with an idea for a potential miniseries – and it’s usually a historical piece. Maybe I see more of these than other consultants, since I'm most known for writing and producing in this format (Band of Brothers and From the Earth to the Moon for HBO).
Whether these writers have written one or more scripts for this proposed miniseries, or just have a treatment/outline for the concept, I usually find myself suggesting that they reconceive the idea as a feature – and I tend to find that most ideas can work as well, if not better, in that form.
I do this because I think scripts for a proposed miniseries generally don't make ideal writing samples. And that's what an unproduced, unrepresented writer is really creating: writing samples. (In fact, that's what we professionals are doing a lot of the time when we write on spec, too – because most projects don't sell or get produced, but they might move your career forward by showing people what you're capable of.) You want those agents, managers, producers and executives who read your work to think, "Even if I don't see this project getting made, I can see this writer working professionally, and I want to be part of it."
The problem with miniseries spec scripts is they don't translate to anything else in the industry. A one-hour miniseries episode does not equate to a one-hour pilot, and it doesn’t show that you could write a drama series. It's not quite a feature, either. And it is probably more open-ended in story than you would want either of those to be. It's unlikely the people you want to impress or convince are going to read multiple miniseries scripts – and you want to put your best foot forward, so that the one script they might read shows you're a viable writer for the marketplace.
Miniseries are also not produced in enough quantity that writers "who can do that" are being sought after. When we made Band of Brothers and From the Earth to the Moon, we hired writers off feature samples. I don't think any of them had scripts for other miniseries that we looked at. They're just too rare. And as far as selling your miniseries idea, the sad truth is that networks looking to make them are looking at established producers and writers (and their agents and managers), not to outsiders - even more than in the feature marketplace.
On the other hand, a good spec feature can do so much more for a career. It's more likely to get produced (and can be independently produced for a viable marketplace), and it shows you can write a good feature – an ability that is much sought after. Sometimes, conceiving something as a miniseries is really a result of not finding a clear story to tell within a historical event, and instead relying on multiple real-life "episodes" as being "the stuff of miniseries." Whether they are or aren’t, I would recommend thinking through the feature possibilities first.
I personally like Save the Cat's ten genres (nine of which can work well for true stories) - and suggest looking at whether your subject matter could nicely fit one of those. Look for what you're most passionate about in this idea, find your take on it as a writer, and see if it can't be a tight, compelling feature instead.
In the end, though, I think you have to follow your passion. And if miniseries is ultimately the best/only fit, and you can write a first episode that knocks it out of the park, then I would say, “Go for it.” Don't expect a lot of interest in reading multiple scripts or a lengthy miniseries "bible," but that one-hour script might be impressive to a potential representative as part of a larger writing portfolio, even though they will likely have a hard time selling it.
In case you’re wondering, I personally have never developed a miniseries on spec, but was employed on projects that were already set up – where there were bigger names involved who made the network interested. When writing on spec, I have always stuck to pilots and features.