What ever methodology you use, EVERYTHING must get done. Its just how you go about it, a dead waterfall project tends to leave quantifiable chunks, wheras a dead agille project tends to leave multiple bits of code that do part of some of the requirements, but dont come together. If both methods are executed fully, you should have a stable, tested, secure and documented deliverable, with all the parameters met, the issue is good Project managers are hard to find, and they usually aren't teamed with good delivery people, so most projects aren't perfectly executed from end to end, and something slips, usually the documentation and security.
HMG and Agile have a culture mis-match HMG havent worked out how to write a contract for waterfall yet, let alone an agile one (incremental payment for incremental work)
Along with the age old problem that HMG dont know what they want from one day to the next (Scope Creep is the no1 killer of large projects) and they are operating on limited funds, with pennywise and pound foolish purchasing departments, which means materials and expert staff are scrimped on, this leaves me wondering how any projects actuall get completed, let alone on time or budget.