iOS 18 is generally stable, but as with most major iOS updates, early versions can have bugs that get patched in subsequent releases. Some users have reported issues such as increased battery drain and occasional overheating with iOS 18, which is common after big updates as the system re-indexes files and apps optimize themselves for the new software.
If your primary concerns are heating and battery life, it might be worth waiting for at least one or two minor updates (such as 18.0.1 or 18.1), which typically address these early issues. However, if you’re eager to update and willing to monitor battery performance and app behavior closely, it should be safe to install now, keeping in mind you might need to tweak some settings (like background app refresh or location services) to conserve battery.