Messages are stored on the server, not on the device
Yes, pretty much necessary to provide multidevice support
end-to-end encryption not enabled by default
True that and telegram sucks big here, but I donth think e2ee can be enabled in a feasible way for multiple devices.
uses proprietary encryption, making security audits difficult
The MTProto isnt open source but its fully documented, there have been security audits on it.
dubious financial backing
No. Pavel Durov have always said since starting he paid for telegram’s servers from his pocket, in recent years telegram has started monetisation programs to cover its costs.
Russian developers
The founders were born in Russia, but they now have dual citizenship of UAE and France. If you are talking about politically questionable, even signal have been accused of having backdoors for CIA.