Doesn't sync with all accounts, doesn't let me input my own numbers (for, example, home value, cash held, assets held that don't have online accounts yet have changing values, like notes, etc.), can't revise / edit historical numbers, etc.
It's just inaccurate. I love Mint, am a huge fan, but their net worth is basically useless for me.
For others with a much simpler financial life it could be a useful start, but I'd still recommend someone move to something like NetWorthIQ (and then Excel), but YMMV, obviously.