The chest mounted holster option has its ups and downs. It evolved out of the massive increase in mobility operations over the last 10 years. Prior to that we always kept our side arms in hip or thigh holsters, but since OEF and especially OIF the average guy down range spends lots of time wearing bulky armor in crampt vehicles. Being able to quickly access a weapon is hindered in that situation, so guys started experimenting with moving it to a chest mount.
The thing is, a fully loaded pistol is relatively heavy and most holsters have a bit of an offset in the mount which can lead to this awkward balance issue when taking a holster designed to hang off the hip and MOLLE mounting to the chest. Kydex and other plastic-based holsters help with that issue, preventing any sagging or flopping around. They also tend to have good retention built in which brings up another point. You may want to think twice if you're out and about pressing the flesh with locals as a chest mounted pistol is an easy target to grab either to extract and use against you or as an anchor point to subdue you.
Throughout my daily routine in Iraq I was in various threat levels and made my gear modular enough to adjust accordingly. Within our safehouse complex we had to have a pistol at a minimum - so M9 on a thigh rig. Within the immediate neighborhood we needed our primary weapon and some protection, so I would grab the M4 and throw on a low vis IIIa vest. Rolling outside the wire required plates, so the PC would go over the vest (SAPI plates back then were in-conjunction anyway) primarily for lighter profile missions. Then for the really bad places like Mosul, the whole vest and PC would be replaced by a dedicated carrier (RAV) which also allowed twice the ammo to be carried.
So back to the chest mounted holster idea. I did most of the driving on my team and it was in a variety of vehicles both tactical and non-tactical, each with its own limitations of weapon stowage and portability. The chest mounted holster was great because there was quick, easy access to my pistol. But once I was out of the vehicle I hated having the pistol on my chest. I also had this two week phase of freaking out every now and then while on foot, taking a knee and realizing there wasn't a pistol on my hip.
Ultimately what worked best for me was to stick with the thigh holster but whenever in the vehicle I would stow the pistol in the RAV cumberbund or a loose area behind a mag pouch on the PC. This of course required a bit of training to holster the pistol quickly while un-assing the vehicle, but that was easy. I did exactly that for the better part of a year with the only change being that watching a trusted friend have an AD on a holstered pistol made me not want my dick shot off, so I would run it initially unchambered.