Budgets, not crashes.
A challenge runs within budgets: how many rules may start a tick, how many steps run before the rest wait a tick, how many mobs, block changes, and explosions the world takes at once. Past most budgets the extra is skipped and counted, and the status says so; past the steps budget the rest waits a tick and goes on.
Heavy rules.
A rule on a frequent event (a tick, a block broken, a move) that searches the world or loops over everyone is the usual cost. The editor's estimate names the rules that weigh most; a cooldown, or a timer every few seconds, often does the same job for less.
The file's limits.
How many of each thing a file may hold is on the reference's limits page, and the editor says when a draft passes one.