Creating a new service node requires a providing a stake in Loki, which I believe currently costs on the order of $5000. And the only source is Loki held by existing service nodes. So arguably, as the creation rate for new service nodes increases, the price of the requisite Loki stake increases, perhaps supra linearly, or even exponentially.
There's also the issue that service nodes that behave maliciously lose all of their Loki, both the initial stake, and anything that they've earned.
I don't know specifics, however. So I don't know just how high the bar is for malicious service nodes.
I get that the Tor Project has banned relays for numerous reasons. The tor-relays list is a good (albeit incomplete) source for reports, discussion and decisions.