Accuracy
The maths is not the limit. The formulae in The numbers are checked against GeographicLib and Vincenty’s published values, and are exact far beyond the digits shown.
Where you put the points is. Zoom in before placing corners that matter, watch the loupe while dragging, and nudge by 0.1 m or 1 ft in the points table. If you have surveyed coordinates, type them in rather than tapping.
Imagery is a photograph. Apple’s satellite imagery can sit a few metres off true coordinates in places, and trees, roof overhangs and leaning buildings can hide or shift a boundary. Where it matters, check a corner against a known coordinate. Measure in a flat style; the 3D styles tilt terrain and buildings.
Long edges. Each edge is measured along the shortest path on the earth but drawn straight on the flat map. Over hundreds of kilometres the two part, and the number describes the edge on the earth; add points along a border you want followed.
GPS. Add My Location and Track My Position are as good as your device’s location, a few metres under open sky and worse near buildings or trees.