yes, really stable OS:
just look in google how may tyros have OS issue and yamaha seem not interested for make update, they prefer to exit with a new model.
If you remember the first tyros 2 shipment in germany arrival broken to the costumers with a fatal error...maybe you forget the old story...

Free to buy what you better like, but don't me make me laugh when you compare the tyros ROM soundfont to the all other bigger company like Native instruments, Gigastudio and so on..

SA is available in some way under gigastudio too, so I don't who really have discover the hot water.
Tyros sounds really good for what they can offer in one embedded system.
Enjoy ( and buy) what you play.