You cannot totally rely on speed signs, Mouche, although obviously they should be obeyed if they exist. You see a speed limit for 60 kph. Is it 60 kph for the next 1,000 km? All roads in most countries have a blanket speed limit, i.e. the limit that applies if there is no sign. The start and end signs for a reduced limit for that particular road are there because there is some hazard, e.g. a junction, a built up area etc.
Suppose you see a 50 kph sign and within a short distance you enter a village. It is reasonable to assume the reduced limit is because of the village. a couple of km later you exit the village and pass the end of village sign but there is no end of 50 kph sign. A pretty common occurrence in my experience. It is now an open road with no hazards of any sort. Do you carry on driving at 50 kph? How long for? The next hour, two hours etc?
The speed cameras in Crete are generally on ΒΟΑΚ. BOAK is defined as a National Highway. It is reasonable to assume, therefore, that the blanket speed limits on ΒΟΑΚ are those for a National Highway as described in the Greek Highway Code. They aren't. They are lower. How is somebody who does not live in Crete supposed to know that? Some sort of information osmosis? That is the main complaint.
The secondary complaint is that many of the signs that do exist are completely illegible including many near camera sites. So yes, people do find it difficult to read some signs including some important ones.