My brother is looking to pick one up. Is there a consensus on brands to stay away from or stick to?

Stainless CMMG is the strongest, most reliable.

Be aware, the stainless ones you find cheap are the old discontinued CMMG.
They do not have the features of the Evolution Stainless BCG's.
Dave N

unless you're limited to one rifle forget the conversion kits ...only good if you're on the run & can't carry two...buy a 15/22 & be done with it ...takecare...Kent

Well, that is part of the object in him getting it. He has 22 rifles, but wants to do it primarily as part of a bug-out-bag (as well as being able to train with his primary weapon for less than $0.30/rd.)
He wants to be able to grab the AR, his bag and go without having to worry about lugging around a second rifle just to use 22lr.
22lr is going to have much greater utility than 5.56 in a SHTF scenario, with the exception of a 2-way range... and you can carry a whole lot more of it for the same weight.

well if he can live with 2.5 to 3in. groups at 30meters & his upper getting full of smoke & crud coming out of the open gas tube then it will work ...the CMMG stainless kit I bought worked good but it's not accurate enough for me evan in a bug-out kit...I tried [9] different brands & no luck ...if it's that ticky then I don't think it useful because how do you know what ammo you can find in the future...I just replaced it with a 15/22 & 6X Leupold scope...have gotten 5 shot 3/4 groups at 30meters with several different brands..what about a 10/22 I've got several that will shoot sub MOA ?...they're cheap but is a different platform that's why I went with the 15/22 to match as close as possiable my M4's...take care ...Kent

Aluminum will win over plastic every time in my book

I shot the 15/22 after lunch today & was impressed it needs a better trigger but at 50 meters I shot 1" groups sighting in a 3.5X10 Leupold....works good on Fed. bulk ammo bought in 1988 with 100% function so I guess the plastic is OK in a .22 ...take care ...Kent

Enjoy it.
The 15/22 was built from the ground up as a .22. It's not a conversion of any type. No reason it shouldn't work.
It's made from plastic to keep the cost down.

The dedicated uppers were developed to use with existing lowers. Actually, the CMMG's were designed as trainers for L.E. and Military allowing the shooter to use their already familiar fcg's and controls. A cheaper alternative when ammo prices increased so much.

Don't put any pressure on the barrel. Your POI will change.
Dave N

The SS CMMG ones work perfectly and are a great value.  Get Blackdog mags.   CDNN usually has them for cheap.

I know the Smith lovers will flame me for this.....and I admit to being old fashion and loving real steel and aluminum...but the best comment I ever heard about that little plastic S&W was at a gun shop right after they came out. I asked the salesman what he thought about the 15/22. And he said, "I think it would be a great rifle for your kid".  End of story. I know some of you guys have them and love them but understand that some people pick them up and just can't connect with them. They are too much plastic and feel like a toy. I am just telling it the way it is for some. It could be the most reliable firearm ever made, be able to group 1/4" at 100 yards, whatever...some people just can't get past the plastic. This is why dedicated .22 uppers sell so well, they fit on a real AR lower and are made of steel and aluminum and feel right.

As for conversions, they shoot better in some rifles than others. I have run many thousands of rounds through them without issue. In some of my AR's they run great for 25 to 50 yard plinking/practice. With decent ammo they could, in some rifles, shoot about 1" group at 25 yards. Certainly good enough to shoot pop cams and golf balls. If you want a target or hunting rifle, don't get a conversion...its not what they are about. They are for cheap practice and lots of fun, not serious work.

Different strokes for different folks. Feel lucky we live in a country where we all have the freedom of choice with an industry free to offer something for everyone.

I tried a .22 upper for awhile & it put so much crud into my lowers that I got tired of taking them to the shop & washing them out ...got to many jingles in these lowers & triggers to mess up...the "kids" gun <LOL> is a neat little gun & easier to clean ...besides the .22 uppers tie up the .223 when I might need them & I need better groups than what I was getting with it...thanks & take care ...Kent...

The .22's do dirty things up a bit....a fair point to bring up. As for the feel,  its all in how the shooter interfaces with the firearm and what feels right to them. Its great that we have choices so everyone can get what is right for them. A couple of years ago we didn't have these options.

So, I have to ask.
Is it just the barrel's twist rate that makes the conversion kits so inaccurate in ARs or does the bore diameter come into affect?
Are the dedicated uppers in 1-16" twist accurate?

Actually, its BOTH.  The bore on the 5.56 is actually a few thousands of an inch larger than a dedicated .22. So, the bullet fits a bit loose. Of course I think this is where ammo varies how well they shoot as some softer bullets or those with a certain shape might seal up a bit better than others.

As for twist, the standard 1-7 to 1-9 of the 5.56 is much faster than the typical 1-16 of the normal .22. Clearly the twist makes a difference. But, once more, we normally find that some ammo reacts differently than others to this twist. It is a combination of many factors including the velocity of the round, the weight of the bullet as well as its shape and how it fits in the bore.

So, we have a lot of factors that come into play. We haven't covered them all skipping things like how well the conversion adapter fits up in the chamber, and more. To make it more simple, you have to try a conversion in your rifle with your ammo to see just how it will shoot.  I found my main rifle was reliable with Federal Bulk, but not very accurate. I also found that the CCI Standard Velocity shot a lot better. Others have found different results with various ammo. Again, no exact answer fits all situations, just that results vary. I can say that I have one rifle with a conversion that with the right ammo would out-shoot my one dedicated .22 upper if I used Federal Bulk in it. In other words, best case conversion could out perform worse case dedicated upper. Not by much mind you...but it was surprising to see.

Sorry, can't be much more clear than that....too many 'ifs' involved.