Sia is very reliable from the customer's perspective. Its only individual systems and storage pools that have lax uptime requirements. Thanks to some clever network-level redundancy mechanisms (10-of-30 redundancy), 95% uptime at the storage pool level translates to 99.9999% uptime from the customer's perspective. See the "Uptime Math" section in the OP for details.
When you says Sia is reliable, do you mean it could be reliable if hypothetical X, Y, and Z things happen?
Because according to the homepage (sia.tech) there are only 895 hosts storing a total of 206TB right now, which is a very, very small amount. Backblaze, a relatively small player as compared to the big cloud providers has 1.1 million TB of raw capacity as of last year (redundancy reduces the available capacity, but still) [0].
By reliable in this context I mean robust against hardware failure. (Including the failure of entire storage sites.) The OP explains the math and associated assumptions for how they derived that "99.9999%" figure, and acknowledges that since the calculated chance of data loss due to hardware failure is so infinitesimally small, other failure modes outside of what they modeled are likely to dominate.
As for the relatively small number of hosts at present, 895 is more than enough for 10-of-30 redundancy to work "as advertised". You really only need 30 hosts technically. The bigger issue I think is the relative immaturity of the software. Sia is still pretty new compared to most other data storage systems; and although I've never heard of any software bugs in Sia resulting in data loss, that doesn't mean such a bug will never be discovered. Be cautious, keep backups, and never rely on any single storage medium to store your data.
Users of Sia are the customers. So your question reduces to: who uses Sia? It used to mostly be interesting for customers interested in reliable large-file data backup on the cheap, but now it has expanded to customers looking to do content distribution on the web, etc.
