Take a look at the docs for the library that powers the Home Assistant integration: Usage — simplisafe-python 2024.01.0 documentation
The unpublished simply safe cloud API has been very flaky of late, so your mileage is going to vary. Some people are having pretty continuous problems; others, like myself, only have an issue every now and again and for the most part, it’s been fairly stable.
You can actually build a nearly complete, totally localized alarm solution all in Home Assistant (using Z-Wave/Zigbee/RF sensors); here’s an example: GitHub - nielsfaber/alarmo: Easy to use alarm system integration for Home Assistant
I say “nearly“ because it misses one crucial piece: a dispatch service. It all depends on whether you want your alarm to merely scare people away with a siren or actually call fire/police/etc.
There’s been some investigation of Noonlight as a programmatic API connected to dispatch services, but my own investigation has been inconclusive: that API is more geared towards corporate partners than individual users, but it may be possible for individual consumers to get keys anyway.