my close friend with a 96SVX has 299k miles on his no issue.
I have seen them in auction and stuff with 455k miles still running fine.
The engine design is not to fail.
keep it oiled and keep coolant, and it will outlast the car body.
A couple other companies use boxer motors, too. Porsche, and both BMW and Honda motorcycles use boxer's in some of their motorcycles.
Subaru is the only company that currently uses boxer motors exclusively in their entire line-up.