ONSITE is definitely an option. However, knowing the platform for many years, I can attest that the maintenance is actually less tricky than with with regular appliances. With scalable, every element is redundant and can be replaced easily. SSM or SGM, in case of failure you just put out the blade and put RMA one in, then the magic happens 🙂 CMM or power & vent replacement is even easier.
What is tricky, that is initial configuration. For that you usually have a jumpstart package where 5 days of onsite PS is included.
Another tricky part is about patching, but it is much better now than a couple of years back.