Management at the home has apologised for the treatment of those in its care after inspectors highlighted concerns.
The nursing home remains closed to new admissions after health regulator, the RQIA, obtained a court order.
Alan Philson of Unison said staff at the home feel under mounting pressure.
“The morale has now dipped very, very low,” Mr Philson said.
He said there were concerns about levels of training, staffing levels and experience among staff.
The 81-bed facility is for people with learning difficulties, mental health issues and dementia.
Mr Philson said staff need to be effectively trained in working with challenging behaviours.
“Staff should have a plan in place that teaches them all the necessary deescalating training, we believe management haven’t given enough support and education to staff.
“Staff are trying to provide the best care they can but feel more under pressure. They are under strain and understaffed and they feel management are letting them down.”
He said he has raised staff concerns with management on several occasions.
Management at the home has said its priority is to address four outstanding issues and deliver the highest possible standards of care.
It added: “We have reassured our residents and their families that they are receiving a high standard of care and this is constantly under review by RQIA and the Western Health and Social Care Trust.”
The home on Derry’s Culmore Road first came under the spotlight in May following an inspection.
Inspectors highlighted concerns about staff knowledge, including nurses not following policies and procedures, as well as inadequate records about areas of patient safety, such as falls, feeding and nutrition.
After a follow up inspection in August, the regulator formally stopped new patient admissions after obtaining a court order – an unprecedented move, according to the RQIA.
Earlier this week, the son of a patient at the home told of the heartbreaking scenes when he found his father “sitting in a his own dirt”.
Michael Stewart said his dad’s catheter bag was frequently allowed to fill with blood and his soiled nappies were not changed.His father Joseph, 76, has Lewy body dementia
Mr Stewart said he felt as if it his father “did not matter” because he had dementia.
He said: “It has just been so degrading for my father, it is as if they think he is not there.
“Of course he is there. He knows what is going on half the time.”
Mr Stewart said he often found his father “sitting in his own dirt”, sometimes for four nights in a row.
“When you try to complain, they turn it back around and say he was found dirty because he was being difficult.”
Mr Stewart said his father was by nature a “loving and caring man” but there was little understanding of the challenges presented by his dementia.
“It has been heart-breaking to watch,” he said.
Mr Stewart said he did not blame carers, who “should not be put in the very compromising positions they are put in”.
He said he had followed all complaints procedures and felt let down by everybody.
“For a home to be running like that, to take on the responsibility of 81 human beings, with all the different things they are coming with, just beggars belief,” he said.
On Monday the Commissioner for Older People, Eddie Lynch, said he could not rule out ordering an inquiry into the Derry home.
He said its failings were “depressingly familiar” to the care scandal at Dunmurry Manor in Belfast.