There are a couple problems with Flowerhorns, as there are with any cichlid or pet of any kind.

Darkening

Darkening is common with Flowerhorns and is an attempt for the flowerhorn to mimic its natural surroundings. All fish do this to some extent. So, a Flowerhorn kept in a tank with a black or dark background will be darker than a fish on a light-gravel background. My fish Bebe is in a tank with a pale brown river gravel. This, plus the absence of a dark background keep her colors at their best.

Keeping Flowerhorns on a black background is not recommended and will result in dark, maroon-colored fish.

Aggressiveness

Flowerhorns are meaner to their rivals than to any other animal or fish. They spar lightly with a tankmate plecostomus and might even completely ignore smaller Flowerhorns, but two Flowerhorns of similar size will kill each other. Therefore, Flowerhorns command a tank of their own, and don't feel bad, they would only kill a tankmate of the same species. Oddly, they seldom harm other types of cichlids or even Flowerhorns of entirely unrelated coloring, for example orange Jin Kangs successfully kept in tanks with a regular Red Dragon.

Fin Breakage

Nets break fins. Don't use nets on Flowerhorns. Use jars, plastic bags, your hands, ANYTHING but a net!

Cold Tolerance

I have read with interest about Flowerhorns being kept down around 45o F however I have had no such luck. I have found that raising or lowering the temperature a mere 15o in a short period of time is terminal for some Flowerhorns. One case was a group of Flowerhorns I moved from 73o to 87o in only 15 minutes. I lost one fish in that group of five. I will not err that way again.

Another case took fish from 72o down to 55o in short order, and I lost one fish out of three doing that. I will not err that way again.

Disease Susceptability

This is not a problem. Flowerhorns are amazingly durable to most diseases. Oh, sure you can make them sick, but it doesn't Just Happen, you have to abuse them pretty much. Poor water quality is the primary cause of disease in Flowerhorns.

Shipping Troubles

See our article on shipping

Breeding

Breeding Flowerhorns is not really recommended. Mostly this is because the rate of compatibility is so very low, you will injure a lot of fish waiting on a pair to form. When you put some fish together, injuries will occur until a pair is established. Fish will die in the process. Based on what they typically cost, this is unacceptable.

On top of that, because of the fractionation of the genetics of the Flowerhorn, your offspring could be AMAZING or DRAB. I don't see much in between.

Seldom do two nice Flowerhorns produce similar Flowerhorns. This is because of the dominant and recessive traits brought by the earlier generations (cross species) in the "making" of Flowerhorn.