I have used P-TTL flash on some events, and have generally been quite satisfied with the results. I don't like the "deer in headlights" look of a direct on-camera flash, so I've only used bounce flash, or off-camera flash. That said, here are some ideas why underexposure might be experienced and how to avoid it.

In exposure modes where shutter time is not specified by photographer, the camera caps shutter time at 1 / (crop_factor × focal_length) when a P-TTL flash is used; if the calculated cap is faster than sync speed, sync speed is used. By this, the camera tries to avoid blur from camera shake, expecting the flash to provide enough light. This cap is applied even if the flash is obviously unable to provide enough light by itself. In Program mode, such cap will not be compensated by using larger aperture. Switch to Manual exposure mode to choose both aperture and shutter speed yourself. (Note that under sync speed, lower speeds will not gain you more available flash power, only the amount of recorded ambient light will increase.)

The flash might simply lack enough power; try using the flash with manual power setting at full power to determine if this is the case. Setting ISO near the capability limit and shooting different subjects while moving around, sometimes the flash will not have enough power. One way that power might become lacking is allowing camera to change shutter speed faster that sync speed; this will require the flash to pulse in High Speed Sync mode, considerably decreasing ability to light. Or the camera could be choosing a smaller aperture than when you were figuring out how low ISO you can pick. Use wider aperture and/or raise ISO to ensure sufficient firing power; switch camera to Manual exposure mode if shutter speeds get faster than your camera's sync speed.

• For calculating distance that could be covered with a flash, knowing and understanding the guide number might be useful. For bounce flash, the whole trip from flash to bounce area and back to subject has to be considered, with some loss accounted in depending how reflective the bounce surface is (say, about 1 stop for a white ceiling). Hint: stadiums, landscapes, and bounce flash in large halls is out of range whatever your GN is.

Pentax metering tends to sway towards avoiding clipping highlights when in doubt; with direct flash, reflectors, mirrors, chromed surfaces and even nearby light-colored objects may cause the camera to be careful and underexpose your subject. With bounce flash, you should be pretty safe here unless some of your flash reaches the reflective surface by a direct route. Use something like a black foamy thing to shield your flash from shining towards scene directly. Shoot in RAW for better ability to adjust exposure in post-production; you can't control the ratio between ambient and flash in post, but for precise control, you should prefer manual flash to P-TTL anyway.

Behavior of matrix metering is hard to predict in settings with varying lighting. You could try using spot metering to take control over what your camera meters by. Automatic exposure also aims for 18% gray, so if you're metering from a lighter surface, dial in some exposure compensation accordingly. Understanding where a subject should fall in Zone system would not hurt.