Active crossovers are not any more difficult to design than passive ones. If you know what you are doing.
But, you have to distinguish between building an active system as a manufacturer, versus doing it as a consumer. If it's the former, it's still easy to maintain control of the slopes, crossover points, baffle step compensation, box tuning, driver selection, crossover selection, even amp selection. One advantage to going fully active for manufacturers is that you can really take a full-system approach to matching drivers, amps, and crossovers. With a passive system, you have much, much less control. Plus, you can use less powerful (and less expensive) amps when you have each one only doing a small portion of the system. The other advantage is you can use steep L-R phase coherent crossovers. Combine this with the ability to time-delay the outputs to each driver so they are time as well as phase coherent, with very little overlap or playing outside of their bandpass, and the advantages for active systems become overwhelming.
However, as a consumer, if you are trying to "rip out" a passive crossover of an existing speaker system and replace it with an active crossover, you had better be doing it with the support of the speaker manufacturer - there are too many variables that you must know in order to get the speaker working properly again. Without direct and specific info from the manufacturer, you are going to be hopelessly lost, trust me.