I'd argue you do have <100% reliability in Factorio, and much of the game is in increasing the 9s.

Biters can wreck havok on your base. Miners contaminate your belts with the wrong types of ore, if you weren't paying enough attention near overlapping fields. Misplaced inserters may mis-feed your assemblers, reducing efficiency or leaving outright nonfunctional buildings. Misclicks can cripple large swaths of your previously working factory, ruining plenty of speedruns if they go uncaught. For later game megabase situations, you must deal with limited lifetimes as mining locations dry up, requiring you to overhaul existing systems with new routes of resources into them. As inputs are split and redirected, existing manufacturing can choke and sputter when they end up starved of resources. Letting your power plants starve of fuel can result in a small crisis! Electric miners mining coal, refineries turning oil into solid fuel, electric inserters fueling the boilers, water pumps providing the water to said boilers - these things all take power, and jump starting these after a power outage takes time you might not have if under active attack if your laser turrets are all offline as well.

But you have means of remediating much of this unreliability. Emergency fuel and water stockpiles, configuring priorities such that fuel for power is prioritized ahead of your fancy new iron smelting setup, programmable alerts for when input stockpiles run low, ammo-turrets that work without power, burner inserters for your power production's critical path will bootstrap themselves after an outage, roboports that replace biter-attacked defenses.

Your first smelting setup in Factorio will likely be a hand-fed burner miner and furnace, taking at most 50 coal. This will run out of power in minutes. Then you might use inserters to add a coal buffer. Then a belt of coal, so you don't need to constantly refill the coal buffer. Then a rail station, so you don't need to constantly hand-route entirely new coal and ore mining patches. Then you'll use blueprints and bots to automate much of constructing your new inputs. If you're really crazy, you'll experiment with automating the usage of those blueprints to build self-expanding bases...