Unforgiving can mean many things, and it depends on what you compare. Unforgiving has a negative note, but can be positive, depending on what you seek.... e.g. you could translate "forgiving" to "less precise". The term is usually used for advanced boards on the stiffer side, which - if you know what you do with your edges - ride more precise, responsive, predictable and stable than soft boards. They'll do exactly
what you "tell" them, enabeling control e.g. when riding higher speed or carving. But the more precise a board is, the more you also need to ride them precisely. They won't "forgive" you akward movements i.e. they'll react to these unintended akward movements as precisely and immediately as to intended ones. If you're a beginner, you don't want a board which reacts immediately to each slight weight shift cos you haven't control over you body n balance yet. You need a "forgiving" board, i.e. one that forgives (=won't react to) slight off balance movements. The more you advance (= have control over your movements) the more you'll seek for a board which translates your precise movements.
Kind of like driving a normal station wagon vs. a Porsche
They also can be unforgiving in terms of being less smoothe in uneven terrain. While you can
cruise pretty stiff legged on a soft board over crud (it'll bend and absorb the unevenness), you're likely to get bucked off on a stiff board doing the same. The stiffer the board, the more you'll need to use your knees as bumpers.
Are you aware of the difference between a carve and a skidded turn? It's kind of essential to judge, if the Custom X would suit you well... If you actually do
carve (rail through a turn a leaving a clean trench, no skidding) would mean that you have pretty good control over you weight distribution and movements. If you'd like to progress that skill (tighter turns, get more acceleration out of the turn, at higher speed, longer deeper perfect trenches), the Custom X would be a sweet board.
The Custom X will be less forgiving than your Flying V (it's camber shape is more catchy, you have to do precise transitions), but compared to other rather stiff advanced boards, it is pretty forgiving cos it still has smoothe dampening in uneven terrain (where e.g. a Volkl Coal will rattle your teeth). If you want a board that carves great and is stable at speed and still pretty smoothe in crud, the Custom X is great. The NS Raptor would be an option as well (that one less catchy due to its hybrid rocker shape and better in pow, if that's also something you want to aim at, but it's bit less precise due to this shape than the camber of the Custom X, e.g. if you want to straight line flatbased).