This is true, but agricultural land, except for field boundaries, largely stays the same. Inland erosion isn't usually (I say usually) significant over 150 years. The constant ploughing of fields does soften slopes over time, it is true, but the slopes do not disappear, hills do not become depressions, and you can usually see, from the nature of the land use, if this might have happened.
Certainly, unless open cast mining is involved, hills do not disappear nor the general lay of the land over a distance of two or three miles – it's 'shape' and 'general nature' doesn't alter much, and that is what I use GE for.
The change I have most often found, is the course of rivers, which can change quite a lot in a short space of time due to human intervention to improve drainage. There was a lot of this in northern Italy, to name but one place, that has changed some Italian Wars battlefields significantly – but again, you can sometimes see the old waterways in the aerial photography. The other thing you can't rely on is woodland – for almost any battlefield.
Most rural settlements don't change much either, they are a little bigger perhaps but, most places haven't seen the changes that, say, the SE of England has. Also the addition of new roads and rail links changes the look of some landscapes, but disregarding the 50m to either side, these don't change the lie of the land much either.
Battlefields I've looked at recently that haven't much are
Marignano 1515, more densely populated, being 10 miles from Milan, but otherwise the general landscape is the same – even the course of the waterways.
Zorndorf 1758, almost unspoilt. Agricultural land.
Bir el Gubi 1941, you can still see the Italian trench lines.
Paltzig, almost the same. Agricultural land.
I wouldn't use google earth for most ancient battles,but it is useful, depending on the local (some battlefields have been completely wrecked by human intervention) for a lot of battlefields, especially if you are aware of the pitfalls. GE should not be dismissed out of hand. Things often DON'T change that much.