greetings.
there are many disadvantages aswell as advantages to having a planted tank, but i've never had fake plants so my view might be biassed.
the good:
a whole new level of aquascaping - plants add new life to the aquarium and new possiblities for creating a dynamic and everchanging look to the aquarium.
fish are more comfortable in the more natural environment that true plants provide.
live plants provide o2, which means that if you have enough plants compared to your stock level you don't need an airstone or another means of adding o2 to the tank. plants also use nitrate.
the disadvantages are:
alot of research needed - lights, substrate, circulation, temperature, fertilization are all essencial for plants, and some plants have higher needs than others. some plants have higher demands than otheres, influencing the costs (which can run high).