It's quite tricky to create HR software that works for the US and UK market, especially when the offering is free and they make the money from selling pensions which will almost certainly be a UK-centric provision.

So while US companies might be able to use it, they'll miss US specific functionality that US HR apps offer.

Source: I built and run http://www.staffsquared.com, a competitor to CharlieHR (but we're not free!)