Talking about these:

Kit with a brace

Someone here had discussed them before, but I am not sure what conclusion the hive came to.

So, legal, illegal, gray area? Experts, please chime in.

Well weight could be the issue here, and in their specifications they've made some mistakes.
The Glock itself has to be on your NY Handgun Licenses (assuming you DO have one)
It IS legal to convert a handgun into a long gun, but not the opposite (which really isn't the case here)
The "Sig-Brace" as most designs have been branded, is legal in NY, trick here is will adding this kit make the Pistol exceed 51 ounces?  If so, it's a no-go.

The weight of the pistol is the same. The kit is an accessory and does not change characteristics of the pistol. I am more worried about it being considered a "barrel shroud".

Doesn't the law say manufactured weight?

True, but there's always the chance of getting jammed up by someone getting too excited over seeing you with it....
After all, it's really a Glock with some accessories attached that's already on your Licenses, not any different than adding other gadgets that make it exceed weight...It's NY: you put down your money and roll the dice.  
Having a well understood argument and sticking with its legality will often suffice questioning unless you've done something dumbassed, then you're better off keeping completely quiet and letting your Lawyer handle things.

Well now that is an interesting thought, and it could be construed in other ways. Let's look at it like this. If you buy the Roni kit equipped with a stock instead of a brace and add it to an already existing Glock pistol, you have just Manufactured an unregistered Short Barrel Rifle.. So the question remains, If you use the kit with the stock, you are now a manufacturer, does the same go for the kit with the brace. Also, Since the stock would change the classification of the gun to a rifle, is the kit in itself still considered an "accessory"? Just some constructive thinking here.

I'm going to error on the side of caution here and say that the kit is no good in NY unless LE with an exemption.

The question that needs to be asked with all of these things is: Can a hostile prosecutor convince a mostly anti-gun jury that what you made is illegal. If it's yes, then don't do it. :) For me, putting your Glock in one of these kits hits these features:

c. Second Handgrip or Protruding Grip that can be held by non-trigger hand
f. A shroud that is attached to, or partially or completely encircles, the barrel and that permits the shooter to hold the firearm with the non-trigger hand without being burned
g. A manufactured weight of fifty ounces or more when the pistol is unloaded

Let's talk about weight. Say, you bought a pistol weighing 40 oz and installed a red dot and a flashlight on it. Now with accessories it weights 55 oz. Is it now an "assault weapon"? I beg to differ.

I agree with you in that aspect however I would not classify this as an accessory. It is an item that can, depending on configuration, legally change the entire classification of the firearm, all be it a rifle or short barrel rifle. this is not the usual accessory. I understand in the configuration you want to do it does not classify as either of those categories, however to undermine the potential ramifications if your wrong, in a state that is very hostile towards gun owners, would be an exceptionally foolish mistake.

The problem is that optics and flashlights have never been considered "assaulty" type features. They're not going to attract attention. A Roni chassis definitely will and once that attention is on you by the people in power who want to make an example of you and aren't spending their own money to take you down, it's guilty until proven innocent. In an interview with a forum member of another site, the asked an upstate NY DA about her views on fixed mag ARs. Pinned, epoxied, didn't matter. If the semi-automatic gun was designed to take a detachable magazine, no amount of "fixing" could make it not a banned weapon if it had any "features". These are the people we're dealing with.

Arguing some points in red.

The Hera Triarii kit is much better quality, and legal as-is in my opinion.

The arguments you're putting forward are semantics and would be laughed at in court in front of a hostile audience. If you look at the decisions that the circuit and appellate courts gave concerning the fight against the SAFE Act, you'll see that none of what you said would fly when the outcome and intent of the law has already been decided against you. That said, realistically, what are the chances of you getting into trouble while having your Glock setup like this? Slim and none. I see the craziest stuff at the range on a regular basis that was never available for sale before the SAFE Act. It's all about whether you're under the spotlight of a cop who has interest in you for something else and this is a nice modifier that they can tack on.

So, you'll be havin' a roni?