Well (and spoilers) it's the Brotherhood of Outcasts. I recently got A Tale of Two Wastelands working, and wanted to show my black-and-crimson comrades some love. Even though, technically, that quest was the end of the Outcasts, I figured that the Brotherhood would have more Outcasts.
But, anyway, a surprise attack, in any event, is usually detrimental. Especially when your fortifications are manned by, what are essentially, upgraded jawbers. The Minutemen haven't had formal military training, but the BOS, and subsequently the Outcasts, have had some sort of rudimentary training.