I think they could and should have had some broad Shetland accents - maybe some of the more peripheral characters, and they could have subtitled them.
When we visited Orkney it took a day or two to tune into the accent, but it was very much a part of the place and the atmosphere - not least because these islands feel more of a connection to Scandanavia than they do to Scotland, (so the people told me), and that comes across in the accent and dialect.
Now when there's anything about Orkney on the telly, I love to hear the Orcadians talk; their accent takes me straight back there

I couldn't live there, though - no trees. We did take a ferry trip to the wee island which does have a wood, and walk in that wood, just to see some!

There are no foxes either (so no foxhounds, so no good for BH

) and I think someone said no badgers either?
One feature of Orkney is that most everyone and most of the livestock are barred up indoors all winter, battened against the almost constant 70mph winds. The final scenes on Shetland were shot against a massive wall of window in the old man's sitting room - would a Shetland house have windows as expansive as that?