It actually really sucked for US citizens overseas, who may have moved when they were children, or when they dodged a draft in the 60s, and never filed their 1040s or FBAR.

The worry recently (as the IRS has been cracking down as of late) is any re-entry to the US with a US passport would lead to a massive tax bill for all foreign income over the past 40 years. Or for Canadians living in Florida who never declared their Canadian retirement funds. Fortunately the IRS offered amnesty in 2008-2010 to most to "come clean and get no penalties".