The Leros engine has commonalities with other engines that have called issues including MUOS 5 I believe.
“During a thorough review, we looked at multiple scenarios that would place Juno in a shorter-period orbit, but there was concern that another main engine burn could result in a less-than-desirable orbit,” said Rick Nybakken, Juno project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California. “The bottom line is a burn represented a risk to completion of Juno’s science objectives.”
https://arstechnica.com/science/2017/02/due-to-concerns-about-engine-juno-to-remain-in-elongated-jupiter-orbit/Operating Juno beyond its designed lifetime comes with a price tag, too. The requested budget for Juno operations in fiscal year 2017 was $39.1 million, which was projected to fall to $14.5 million in 2018 as the mission came to a close. Now, if NASA must come up with an additional $100 to $150 million for an extended mission, those costs will almost certainly harm other missions in the agency’s science directorate.