I have an FCPC V5. I use an AXL Equinox Cummerbund on it with VXV Concepts adapters to connect the cummerbund to the front and rear plate bags.
I have the Ferro Concepts side plate bags, they connect via hook and loop to the inside of the cummerbund.
I have the the SHOTSTOP III+ G2 side plates. I went with these plates due to the level of advertised protection and the fact that they're very thin since I didn't want a lot of bulk on my sides.
Honestly, I really like them. They curve well to my body and basically hugs my body. The weight is not bad at all with these plate and my Hesco plates. I do NOT have any pouches at my 9 o'clock and 3 o'clock position on the cummerbund so the bulk is not bad at all.
If you are a civilian and you live in CONUS should Heaven forbid anything catastrophic happen chances are you'll be at some point inside a normal soft skin vehicle with your plate carrier on. American infrastructure and zoning is all centered around roads. Unarmored vehicles, i.e. just normal cars, have almost zero protection with their thin skin doors so side plates can be important as you might imagine. When you move through a building interior there will be times when your sides will be exposed to unknown potential threats. If you found yourself in a densely populated area with a lot of man made structures (cities, suburbs) you're basically surrounded by hard man made surfaces that will all be risks of ricochet, spalling, and generally flying debris. One thing the movies never accurately show is how dangerous such threats are, I have personally met one former police officer who was crippled by a ricocheted 7.62x39mm round fired by some druggy's AK, the bullet skipped across an asphalt drive way and struck him in the leg ending his career and forcing him to walk with a cane for the rest of his life. So, having the ability to scale up your armor is kind of important if you're going to kit yourself up to a 2A loadout or if you're just even contemplating having to bug-out in your normal civilian vehicle. That's my thinking on the topic anyways, and too each their own.
In terms of negative, the only real negative I have from it is that my cummerbund has to be adjusted for the side plates and even with the VXV concepts adapters it's still a PITA to adjust the cummerbund, not something that can be done quickly. So, I have my plate carrier set-up for the side plates most the time. Also, keep in mind my experience with side plates is limited to exact the configuration I describe above and my body. Everyone is built differently and different plate carriers with different cummerbunds and side plate bags may equal a much different experience than my own.