The phrase "are difficult to maintain" is correct and usable in written English.
It can be used when discussing challenges or complexities associated with keeping something in a certain state or condition.
Example: "The old machines in the factory are difficult to maintain, leading to frequent breakdowns."
Alternatives: "are hard to sustain" or "are challenging to uphold".
Contact sessions with Jesse are cancelled at short notice and routines are difficult to maintain.
"It needs a set of very strict controls in place and the standards are difficult to maintain.
"Frequent rituals, like saying kaddish every day, are difficult to maintain, and without strenuous effort they cease to be meaningful," Mr. Sarna said.
Cryogenic propellants are difficult to maintain in a liquid state; they tend to become gaseous and "boil off" of the vehicle.
But such improvements were found only in conjunction with other changes in the subjects' overall lifestyle, like regular exercise and stress reduction, and such changes are difficult to maintain over a long period.
Another casualty of the changed landscape can be sports programs, which are difficult to maintain when different schools run on different schedules — and some small-school leaders shy away from joint activities.
Because Mozaic is one of the few fine dining restaurants in an isolated place, service and cooking standards are difficult to maintain, and the chef is known to drill his staff with classroom lectures, role-playing exercises and even graded tests.
One result of such scholarly research is the realization that the theory and practice of sacred kingship in a history extending over thousands of years—has undergone immense changes and, thus, because of these widespread and extensive differences, all generalizations and categorizations are difficult to maintain.
Systems are difficult to maintain.
However, such models are difficult to maintain accurately and efficiently.
Systems are difficult to maintain and control due to log heterogeneity.
