What are the advantages and disadvantages of doing a census? Surely the more data the better

Although taking a census is better for getting the most statistically accurate results, a census requires taking data from the whole population; which is time-consuming and expensive.

You're absolutely right, the more data the better, which makes a census the most accurate type of data collection! However, logistically, it would be extremely difficult to conduct one; it would be expensive and time-consuming.

A: Greater Accuracy, more representative of the population.

D: Time-consuming, expensive.

Advantages:

Better representation of the overall population

Leads to more accurate results

Better predicting power

Disadvantages:

Costly

Time consuming

Hard to manage/organise

Often taking a full census will be impractical. Imagine if you want to know the mean or median mass of an adult starling in your area. Starlings flock in the thousands, so catching and weighing all of them, even a single flock, would be time and cost prohibitive, when you could get a pretty good idea by catching a hundred or so at as close to random as possible.