There has been alot of debate on the side charging(S/C) uppers..
I own 2 setups myself and doing a 3rd.
As for your "gunsmith" he doesn't have a clue about them.Nor did he sound like he wanted too..
Your "gunsmith" mentioned not accurate enough??
The S/C setup was actually designed for Long Range AR's.
I know plenty of S/C rigs that shoot 1/4 MOA all day..look at one of Alexander Arms top of the line GSR rifle.
Alot of long range guys like the side charging handle as it adds a little more weight to the carrier, and they can also charge the rifle from the bench without having to pull out of the scope..F-class guys will use a single shot bob sled and manually load the round..making it easier with the S/C.
World Famous shooters Glen Zediker and David Tubb have build some incredible AR's ..
Look and see what some of their Precision AR's have for an upper
Your "gunsmith" also mentioned not rigid enough??
Maybe not rigid enough for a tank to run over it.Look around you will see plenty of 6.5 Grendel/6.8 SPC builds using a S/C.
The grendel is about as hot as you can run in the AR-15 platform anyway.
The MEGA upper/LAR upper are beefed up for the fact that more material is removed for the pocket.
As for my downsides.
1)more of a chance to get dirt in the carrier.
2)one more thing to tighten/add when you assemble/disassemble rifle.
As for the ASA Upper..cannot say yay or nay on it..personally i am not interested in it..too many parts in the handle.
Just my take.