I'll put it out there at the risk of having my head bitten off by the friesian lovers.
They are cart horses. Not many make great dressage horses, especially at FEI. Sure, they look flashy and can bend their joints. But unless you get a REALLY special one, their hocks are out behind and they have no extension to speak of. Instead of lengthening the stride, they just come higher.
They are naturally leg movers, not back movers. So again, flashy legs, but their back stays stuck like riding an ironing board.
Nice horses if you want to play in the lower levels and get good scores for having a fancy horse (have to love judges who can be blown away by black, hairy and fancy), but FEI they tend to fizzle out. As I said, you can get the odd one, just like the you the off arab, thoroughbred, quarterhors, morgan etc etc etc that will climb up the ranks. There's a couple of FEI friesian stallions here in Australia, but they are very VERY rarely in even the top 10 placings of a small field. And if they are, it's not usually in a big competition.
I'd take a good look at what you REALLY want. Do you want to be competitive in the FEI levels, or would you prefer to have a really lovely looking horse to have some fun on but not be immensely competitive? If you want to be competive.... find a black warmblood - best of both worlds :P