I feel like halfway into this thread, deep went from being past the service line to within 4 feet of the baseline.
The problem I see with hitting deep beyond maybe the 5.5 or 6.0 level is that players at this level can easily use that extra power and use it to attack you with unless you put them on the run. 5.0 players hit every damn shot deep, so it obviously isn't an inherently bad thing to do. If you look at that video, a good chunk of the harder hit shots were hit off of the deeper balls. It's the same concept with throwing hard serves and fastballs. If it's right in their strikezone, you're going to get burned for it. When the ball lands shorter, especially if they don't feel comfortable coming forward because 1) you're comfortably recovered or 2) they don't want to play the net off of anything but a free ball, then hitting shorter is safe and it makes them hit up, because by the time the ball gets to them the ball is dropping, which means they need to lift the ball over the net. If the ball is on the rise, they can just hit through it because the deflection angle of the ball will be upward. This means more work, more potential ball speed, and most of the available racket head speed, is spent getting the ball over the net and applying the topspin to keep it in the court rather than blowing it into a corner at 90 mph. Think about it. You can only swing so fast. If the racket is going more up than forward, you lose ball speed. That's why flat shots are faster than topspin shots. It's not because the topspin slows it down, it's because your available racket head speed was spent generating spin rather than pace. This is why I struggled with hitting balls after they dropped for a short while after almost exclusively hitting my shots on the rise. When hitting on the rise, I just needed enough spin to drop the ball back down, and I didn't need to spend much energy to lift the ball. When I let the ball drop, I had to spend more energy lifting the ball over the net as opposed to hitting it through the court. This is also part of why when you get a short ball, you should take it as early as possible. It's simply much more beneficial to take it on the rise than when it drops. Yes, if you let it drop, you have the option of holding the ball, but you're also going to hit a weaker shot. Depending on the shot, the difference in ball speed might not actually be that much (usually the balls that land short and don't bounce as high), but the amount of time you take away from your opponent more than makes up for it in these situations. In every other situation, the difference in ball speed is quite noticeable.
It's comparable to table tennis. The serve and first several shots thereafter are all very short slices. The instant a ball pops up or goes deep is when the players start attacking because that's the only time they can take a full swing at the ball. Here, it's more that you have more pace to work with and a better deflection angle which eliminates the need to lift the ball over the net.