Not being a legal expert, I would expect that whether or not you could be sued would have something to do with whether you've completed your role (responsibility) in good faith. Don't do a pitch inspection and 2 minutes into the game somebody breaks an ankle on a hole in the ground? Or, in the aforementioned icy conditions, don't do any pitch inspection at all?
Of course anything can get tested in court, but I would argue that stating it's impossible for a referee to be held liable for anything would be reckless. However, as long as you've done your job then you've exercised your responsibility within the laws.
Given that no jewellery is permitted on the field, for instance, then I would imagine a referee who has acknowledged jewellery but said he doesn't care about it (and yes, it happens), then have a serious injury occur as a result, could have more to answer to than anything that has been mentioned here. Not saying he could be sued, but I would imagine that Law 5 would only protect the referee insofar as the referee is found to be doing the job. If a lawyer successfully argues that the referee hasn't fulfilled his requirements and/or Law 5 contravenes the law of the land, then there could be a problem.
Basically, do your job and I can't imagine you could face any problems. If, for instance, you're in 2 minds about the condition of a pitch but play it anyway, don't tell people that you're concerned about the safety of the pitch but you know the lads want to play. But of course, that's on par with jesting 'extra time? I don't do that - I make sure there's a penalty!'
I personally would prefer a referee be aware (perhaps incorrectly) of the possibility of legal action if he isn't doing his job, versus a referee (perhaps incorrectly) be told that no matter what he can't be sued.
Because ultimately, stating that a referee couldn't possible be sued is providing legal advice. And is anybody on here capable of providing that advice?