The short simple answer is that if you are traveling, any knife you carry may be illegal at some point. Too many laws, at too many levels, is the first problem. You can be in full compliance with Federal law, and still run afoul of state law. You can be in full compliance with both of those, and still run afoul of county law. You can be in full compliance with all three of those, and still run afoul of local ordinances. And finally, as if that were not enough, you can comply with all of the above and still manage to violate the rules of a particular building or area.
The second problem is the vague wording of some of the laws. Some of this is probably accidental, a case of legislation written by people with little or no real knowledge of the item the intended to regulate. But at least some of it is intentional, to give legitimate leeway for circumstances. Phrases like "other dangerous knife" and "with intent to do harm" fall into this category. This is not alway unreasonable, a fifty-five year old businessman, dressed in a suit and tie, sitting on a park bench at lunchtime a hundred feet from anyone else, calmly peeling an apple with a Spyderco Civilian may not draw a second glance from a passing LEO, while a twenty year old, dressed in current "gang-banger" fashion, walking down the street at night in a crime ridden neighborhood while opening and closing a Meerkat repeatedly might very well get his fingerprints and mugshot taken. The danger here lies in the fact that there are thousands of shades of grey between these two pretty black and white extremes and just where the line falls between question and don't question, arrest and don't arrest, indicte and dismiss, or convict and acquit is not always going to be either clear or consistent.
This brings us to the third potential problem, these laws are not enforced by machines which all think alike, they are enforced by people. Four different LEOs may see the same thing, one may ignore, the second question and release, the third question and confiscate the knife, and the fourth arrest you. Even the same LEO can take different actions on different days. This incosistency may be caused by something as irrelevant as what kind of day the LEO is having, or something as relevant as an informant's tip that a gang war is in the making. Even something as seeming cut and dried as blade length can be suject to interpretation, I showed the same knife to several LEO's and asked how long the blade was, some measured the way the manufacturer did, edge of the handle to the tip, but others measured only the sharpened edge and came up with a shorter blade (perhaps even more interesting was the fact that none of them had any accurate way of measuring it, I had to supply the ruler too, which leads me to think, once again, that a call in the field would be more subjective than objective). DAs, judges, and juries are also not one hundred percent predictable, so if you are arrested, the outcome in all but the most obvious cases is anything but certain. But do keep in mind that, to the average non-knifeknut on a jury, a neck knife or tactial folder is going to raise a whole bunch more red flags than a Case stockman. Like it or not, fair or not, that's just how it is.
The final factor is the general demeanor of the person who is carrying the knife. Some folks can hail a NYC cop and calmly ask for directions while carrying an unregistered handgun, others would be sweating bullets and squirming worrying that the two inch slip joint locked in their briefcase was going to get them set away for life just because they saw a squad car cruising down the block. Attitude can make a great deal of difference when it comes to who gets questioned and who doesn't, and that's really the bottom line because if you avoid the initial questioning, you avoid everything that could come after it.