I actually think it's possible that full Watcher + full JK versus full Ninja + full Strongman is townsided. If the same member of scum always does the kill, we have a relatively balanced setup (town have one useless power role, plus one full-powered power role that balances the game by itself). As such, if the scum mix things up, it's likely that the game will still be closed to balanced, because we're changing between two power roles with fairly similar win/loss balance every night. Once one scum dies, the town are aware of which power role will work, but that doesn't change things much. However, what
does
help out town a lot is a massclaim; when both town power roles claim, the setup should be fairly evident (especially if a scum has died or there's evidence of an action being interfered with, like a Watcher watching nobody kill their target, or a Jailkeeper's target dying). That's an advantage they don't have in the 1-power-role setup.
However, there's a fairly nasty twist here: once the Ninja dies, if town don't predict the existence of a Strongman, then they may end up accidentally confirming the Strongman as town via the JK action ("I blocked player X but the kill went through anyway, they must be town"). That's something that adds a notable amount of skew (i.e. onesided swing); in most cases nothing will happen, but we get a very lopsided result if that (fairly unlikely) situation actually comes up. Given that Watcher is highly swingy anyway (based on whether they hit), and Jailkeeper is also somewhat swingy (the role gets more powerful as the game goes on, so there's swing based on how long it takes the Jailkeeper to die), we're talking a pretty nastily swingy setup here, possibly too much so.