Pros: It would be side charging, which I guess some people care about.
Cons: You are introducing new designs to the rifle that have not been tested even remotely as thoroughly as the existing 60+ year old system.
You are also introducing new parts that may not be easy to repair or replace. There's really no standardization of parts either. If the place who designed your side charging upper goes out of business, you're SOL.
If it's just a range toy, go for it, or buy one of these instead. At least then, you are not married to a side charging system.
As a left handed shooter I like the design of side chargers where the handle is mounted into the side of the bolt carrier on the right side of the firearm. They play well into my love of shooting right handed bolt guns.
There are a few different replacement charging handles like the one linked above that keep the handle on the left side of the rifle.
None of them are really needed honestly. You can always get a bigger/ ambidextrous latch if optics are an issue with a standard handle.
I bought one. The only downside I realized after buying was it's a PITA to remove the bolt. You have to use an Allen to remove to charging handle for it to slide out of the receiver. There is also a threaded plug where the charging handle normally is that has a flat side that has to be aligned perfect to close the upper onto the lower. Not the biggest deal, but was like SOB when I went to clean it.
The bolt seems standard, just drilled and tapped for the side charger. Not terribly hard to recreate if needed.
It is a 7.62x39 upper so the side charging keeps it AKish.
Bear creek had a dual charger (side and rear) upper for a while. I thought they were pretty clever and offered some redundancy.
A problem I’ve seen occur with side chargers fixed with an Allen bolt is the bolt backing out. Solution was simple, longer bolt and some blue loctite. If it had been a time you were dependent on your weapon though, it was fail.
A pro that I’ve been using the side charge only uppers for are dedicated suppressor weapons. I wasn’t a fan of the gas that gets blown back at my face through the rear charging port and side chargers do away with that.
I have a gibbz upper that uses a standard bolt with modified cam pin added. I think it's pretty damn cool. Nothing weird required to disassemble it. You are sacrificing forward assist which could either save your life in a certain situation or just jam your rifle up so bad it's unusable.
Mine also runs a piston so it feels punchy like an AK but if I want to suppress it the adjustable gas block would be handy. When I first got mine the detent was so weak the handle would slide back after a few rounds. Since it's non reciprocating it's no big deal. A little turn of a screw and that's remedied.
Since I'm used to a reciprocating handle like my VZ 58s run I would consider one of those too. Not a deal breaker for me. The gibbz design is solid, the dudes that made it are former aerospace engineers and I see it running fine for a long time.