Most advice that a lawyer gives is subjective; facts are objective but opinions are always subjective. What a lawyer does when they advise a client is typically called a "legal opinion". The reason it is subjective is that, as Dale M said, there are numerous variants that go into an opinion, and reasonably trained professionals (attorneys) can disagree as to the outcome of a specific factual predicate. Often times, case outcomes will differ based on the application of the facts to the law, so much so that the case outcome can differ based on the choice of words a witness uses, or even the way a judge interprets the law.
This is why unlike truly objective discipline such as mathematics, where there is a right and wrong answer, no lawyer can ever say a case will definitely go one way or another. It will always be dependent on perception, which is the very definition of subjective. So, whoever indicated that lawyers don't give subjective advice was simply misinformed. They do. What they try not to do is make value-judgments, saying that things are good or bad; rather, they are trained to indicate whether something is illegal or not, or likely to get you sued or not. However, these are all legal opinions.