The only harm the census does is racially categorize people. This wouldn't be a problem if the government waned from its 1870's definition of "race". I will show how this is a problem in the following example: I am Mexican (not really, but hypothetically), I am so Mexican in fact that I actually know what tribe I am from. According to the Unites States, because I can actually claim my tribe, I am considered a native American (not many people would disagree with that, unless they think all native Americans have fucking feathers in their hair). However, if I do not know my tribe, even if I can trace my lineage back before the fucking mayflower (let's say the tribe's name was lost), then I cannot claim to be a native American; instead, I will now be labeled "Hispanic". For all intents and purposes, I would be labeled as white.
How so you ask? Well here is the literal play by play that is stated:
Hispanic.... Hispania... Spain. Spain is European. Yeah, Spain may have the language going for it, but the only link to Mexico, Brazil, America, etc had was that Spain was the main Imperial power for an extended period of time in those countries respective histories. Granted Spain had a massive influence, but if you ask those peoples today if they are Spaniards, you will get a resounding "No!". The government doesn't take this into account.
You can look at this in two ways: 1. No big fucking deal, it's not that important - to which a proper response is "why bother doing a census on the ethnic categorization of the peoples of the country if you are not going to do it right?", 2. Many people get pissed because said labeling on a federal level seems to really look bad when considering how the labels themselves are also indicative of culture (i.e. mexican culture, "black" culture, "asian" culture, et al whatever).
There are other things, but whatever.