Since I started on the railway the headcount of station staff seems to have increased exponentially. Unfortunately the use of agency staff means the usefulness of those staff has gone in the opposite direction. Mainly because most of them are agency with little interest/motivation in the job. It's also meant you can't cross cover as each individual task is carried out by a separate entity. I've long advocated that the railway should follow the Aldi/Lidl route of paying more but expecting more including covering multiple roles where appropriate.
You're a little mistaken with agency work as a lot of them can also cover passenger assistance and other various customer service roles. I don't think safety-critical roles are covered though, but these often have spares ready anyway.
But yes, agency staff can be unreliable or poor, primarily as they may be unfamiliar with layout of the station or the service pattern/lines available. It isn't too bad when agency staff are paired with a TOC staff member though, and some agency staff I have met are absolutely fantastic at their jobs.