Having all of the rope needed for a pitch stacked onto one's harness does increase climbing difficulty a bit, but it is surprisingly less awkward to climb with all these loops hanging past your feet than in looks. With a bit of practice, one gets good at pushing the rope out of the way midstep with a foot, similar to crack climbing where the rope always tries to run into the crack. Of course, rope soloing is always going to be a bit more awkward and higher effort than a live belayer.
Why not the Silent Partner?
While working as an engineer at BD, we had a Silent Partner around which had been disassembled to inspect the internal mechanism. The seat-belt style brake system relies on very small toothed steel gears which swing out with rotational inertia and jam against the inside of the main aluminum housing. These devices seem very robust out of the box as a trustworthy catch, but to me seemed as if the steel to aluminum interface would deteriorate over repeated falls, changing the locking characteristics. This is simply my interpretation of the mechanism, but I trust the GriGri much more in the redpointing scenario of many falls as there are no crucial metal-on-metal catch surfaces. I have used a Silent Partner for fast aid climbing, but was hardly ever falling on the device.
How Harsh are the falls?
I liken this setup to getting a very loose belay (extra slack in the system) but with a hard catch. The loose belay aspect doesn't affect the fall force too much, as long as factor 2 falls onto the belay are carefully avoided. Though, the extra slack makes this system slightly higher risk for climbing above a ledge or cragging with the ground nearby. The hard catch can be reduced while wall climbing by feeding the rope down from the anchor biners, through a locker on the haul bag straps (be sure the haul bag tether is secure as its own system), and then up to the first piece of pro. The haul bag is pulled up in line with the rope during a fall, absorbing some of the fall energy. On Father Time, I took many falls working out the crux pitches, all fairly long, but soft with the haul bag catching me!
What are the pitfalls of this setup?
Rope soloing is all about trade offs, and this method does have a few of its own. The main risk is the rope feeding through the device and spooling up on a ledge, adding a ton of extra slack to the system without the climber knowing it. This can be avoided by keeping pitches short or by using sacrificial rubberband prussiks a few times per pitch to hold the rope weight. The smoother the rope is self feeding while climbing, the higher the risk of back feeding, so it can be difficult to find just the right rope to use.
A similar risk is increasing the length of a fall due to the "brake" strand having minimal weight. Particularly, if a climber falls while the current cache loop is very small, the entire loop may feed through the device before it locks up. Again, the system causes longer falls in general making it less suited for cragging or stepped terrain.
The last quirk of note is the fact that so much rope is stacked onto one's harness while climbing. I've found this to be reasonable for 35m pitches and under, but it may not be the optimal setup for rope soloing El Cap in a day, or other scenarios where long pitches are needed.
Why bother soloing?
I only go lead rope soloing a few times a year, but these ascents are continually stand out experiences. To me, climbing is about improbability - from improbable positions to unlikely movements. And what is more improbable that walking up to a huge wall with a backpack of rope and climbing to the top?