There are two core truths concerning game development:
Everything always takes more time than you’d expect.
If you expect things to take more time than you’d expect, Truth 1 will do its damned best to hold firm anyway. Looping logical paradox may ensue.
Flippancy aside, game creation is generally a volatile space, even in its best-case scenario. Even being part of some AAA project in the heart of On-Rails Corporate Gametopia packs a degree of uncertainty, challenging time estimations and system-crashing curveballs. Moving towards the other end of that spectrum (hyper-indie, underwear-in-basement noodlecoder) brings more of these problems forward and becomes one of the main obstacles in everything “going as planned”.