Not everything should be done in a field. Knowing which jobs travel well saves both money and a second repair.
Jobs that belong on site
Hose assemblies. The obvious one, and genuinely better done at the machine. The assembly is measured against the actual routing rather than a catalogue, so the length and fitting orientation are right. See hose repair.
Fittings and adaptors. Quick, no cleanliness issue, no reason to move a machine for it.
Diagnosis. Pressure and flow testing is far more useful on the machine in its working environment than on a bench, because faults that only appear hot or under load will not show up in a workshop.
Coupling and attachment problems. Almost always a same-visit fix.
Jobs that really should come in
Cylinder rebuilds. A cylinder is a precision fit. Opening one up outdoors means dust and grit going into the bore, and the new seals will not last. There are exceptions, an accessible ram in a clean workshop-like environment, but the default should be to bring it in. See cylinder repair.
Pump and motor strips. Same reasoning, more so. Internal clearances on a piston pump are measured in microns.
Anything needing the system flushed. Proper flushing needs equipment and somewhere to deal with the waste oil.
The middle ground
Often the sensible answer is both: diagnose on site, then decide.
A large share of the cylinders we are asked to reseal turn out not to need it, because the drift is coming from a valve rather than the piston seal. Establishing that at your yard takes twenty minutes and saves removing a cylinder that was fine.
The other middle-ground case is getting a machine mobile rather than fully repaired: a temporary measure that lets plant be moved off a highway or out of a live area, with the real repair planned properly afterwards.
Avoiding paying twice
The expensive pattern is a mobile repair attempted on a job that was never going to work in the field, followed by the workshop repair it needed all along.
That is avoidable with an honest conversation before anyone starts. If we think a job needs to come in, we will say so at the point of diagnosis rather than after billing you for an attempt.