The main problem I found with psyllium capsules were that they did not disperse and usually I wound up passing little balls of psyllium gel instead of the long ropey material that comes from taking the drinks. The capsules tend to dissolve and only the outer layer of psyllium powder is allowed to soak up fluid and swell. The rest of the powder remains dry. I consider the psyllium capsules to be worthless for serious colon cleansing. Although the drinks are a hassle to prepare and a challenge to get down, they are well worth the effort.
By the way, psyllium does not digest. The gel capsules dissolve and since psyllium is mostly an insoluble fiber it does not digest, it goes through the digestive track swelling with absorbed fluids and sweeping the walls of the intestines clean, the added bulk makes bowel movements larger and more frequent without stimulant drugs.
Tghe only good reason to use the capsules would be if you were at work or someplace where you could not mix the powder. Even then you will not be getting the full advantage of psyllium when it is suspended in liquid.