This can be quite accurate. Sometimes they are the only way to do an application. These days they are an excellent trade of since there cost is low and the accuracy is high. However if you need to measure a lot of things they can be very slow. If you want to count large numbers of things they may be no help at all.
Automatic systems are very fast.
They can count thousands of items in seconds. These counts can be linked to sizes and statistical analysis done at modern computer speeds. This sound like this is what every one needs but there are problems. What they do they do well and what they don't they don't do at all. Fully automatic systems use the density or color of the specimen to automatically measure multiple points in the specimen.