Yes - a series resistor will only work as a "constant current generator", when a significant percent of the voltage is over the resistor compared to the voltage over the diodes. So a 5% change in diode voltages (say from component variations) will still not make much percentual change in voltage over the resistor and hence a small percentual change in current.

As the percentage of the voltage that is over the voltage is reduced, the amount of variation in LED voltage or in supply voltage you can handle also shrinks. There really is a big reason why there are so many constant-current solutions on the market. Active current regulation gives big advantages. Or actually huge advantages. And a real constant-current solution will manage to keep the current even if one LED should short. So you don't get an increased current resulting in an avalance effect through all the rest of the diodes. All you got is a potential for a higher voltage loss over the CC generator. And a good CC generator would contain a temperature protection making it deactivate in case too many LEDs have failed resulting in a too high amount of voltage for the CC generator to swallow.