This may be a little wild, but what are the downsides to doing some
combination of all of the above?
A mixed strategy could reduce ... the upfront cost of buying and hosting
hardware (#1), the ongoing costs of renting (#2), and dependence on the
generosity of a third party for sponsored hardware & hosting (#3).
It seems like any strategy should have some redundancy (e.g. multiple
independent build farms) so that a failure in one datacenter does not
effectively take down the whole network...
That would be my opinion too.
But for the cloud renting I would first research if there are associated
network or other costs, because the computing is cheap only to lure
you into the (sometimes prohibitive) hidden costs.
... though they are not full replacements for each other.
Maybe that should be treated as a bug/issue.