Part 1. Part 2 here.
Double Agent is definitely the most interesting game in the series when it comes to its development. Followed by Conviction and its scrapped social stealth homeless bum simulator.
A lot was happening in the industry at the time and Double Agent from its concept down to its mechanics reflects that. So, everybody knows that Splinter Cell Chaos Theory is not only the best Splinter Cell, it's the best game ever made and as such Double Agent had a lot to prove.
With 94 on Metacritic, Chaos Theory was the most bang for your buck game of the series with the most deep single player campaign, a 2-player co-op mode with a separate story and an expanded PvP multiplayer with Spies vs. Mercs. It pushed the last drop of power left on the original Xbox to produce some of the most impressive visuals on the platform. On PC the game still holds up thanks to Shader Model 3 effects and Hi-Res textures which enhance the graphics even further pushing them into the early 7th-gen tier. All that considered, it sold fine, but not MGS 3 levels. Something had to give, with the amount of investment into the last entry, the returns were not high enough, so radical changes had to be made.
Chaos Theory on PC:

Unlike the previous game, this was developed not by Ubisoft Montreal, the original creators, but by two studios, Ubisoft Shanghai and Ubisoft Milan. Both were also responsible for Pandora Tomorrow so they had the experience.
The setting of the game goes in an extremely different direction. Gone are the days of techno-thrillers, convoluted geopolitics and black-and-white story arcs. The new era sees Fisher becoming a double agent - working from within a terrorist organisation.
The terrorists themselves don't have much depth, they are called the John Brown Army and they basically want to nuke US to destroy the current order of things and then make the country great again. In order to keep his cover Sam will have to obey the orders of his "colleagues", which are almost always highly illegal and immoral. On the other hand he has to make sure that the NSA trusts him, which forces him to spy on the terrorists from within, risking his cover.
This is an interesting premise that does go against the overall Splinter Cell theme somewhat but in theory it would bring enough new and exciting concepts to the table.
In actuality though, the game is not that impressive. First of all, the tense balance between maintainig cover with the terrorists and trust with the NSA is not tense at all. You have a Trust Meter that has two lines that represents both factions.

Each mission gives you multiple objectives that allow you to boost your relationships with both factions. Let's say you are completing a level where main objectives are from a terrorist side. Naturally, you are inevitably going to boost your terrorist trust. Simultaneously, you can do side stuff for the NSA like planting a bug / hacking a PC. When you do encounter a choice that will inevitably hurt your trust with either faction, there's always a happy resolution. You foil a terrorist attack? Don't worry, just set another person up as the fall guy. You don't foil it and thousands of people die? Just hack some PCs later and regain your trust with the NSA. You fail an assasination? Not to worry, just lie about it! And it goes on and on. There are no real consequences to your actions. And of course there aren't. Why would anybody play a game that "game overs" you over a choice? The concept is just not applicable to a video game.
The levels themselves function much like the levels from previous games. They are linear, though they provide some type of an alternative way in, like instead of going through the door and attracting attention, you can use a vent to climb through.
One massive addition are the JBA levels that take place on the terrorist base. There you can acces certain areas freely and some you have to inflitrate stealthily as you don't have the permission to enter. The idea is to collect info on the terrorists while completing tasks from the baddies like assembling bombs, disposing of dead bodies and running obstacle courses. It is here the designers were able to go wild with the concept. Unfortunately not much is that different. Sure, when you first get to walk around the JBA HQ, it's quite interesting because you can listen to conversations, scan fingerprints and break into personal quarters of the group's members. But at the end of the day you still crouch, lure guards out and turn off lights to pass through. The only difference is that you can't kill or knock anyone out, which makes sense. How else should a level like this play? I don't know, but they don't feel as fun as normal levels because of how restrictive they are, sadly.
The core gameplay has also been modified. There are more lethal gadgets at your disposal now. Mines, frag grenades for your SC-20K launcher, even the sticky cam can explode now. A entire stage set in a war-torn region in broad daylight is added. Stealth is still, always possible and open combat is never forced on the player.
A lot of gameplay elements were taken from Chaos Theory. NPC Animations, their AI, even the voice over. Unfortunately, what worked very well in CT, is very buggy and inconsistent here. Sometimes NPCs don't react properly to noises, they either ignore them or become hyper-sensitive to even the quitest of movements. It's a little difficult to explain, but with a more complex level layout the AI was not properly upgraded and they still behave like on the original Xbox. The lighting system has been completely reworked by making Sam completely hidden in the shadows and 100% exposed when not. Visually, shadows have become less pronounced and realistic, making the player seek out where exactly the supposed shadow hides the character.
You're are invisible here (hence the green light on the back):

Whereas here you are not hidden at all:

Perhaps the lighting was changed to showcase improved graphics and they do look impressive. The year was 2006, PS3 was yet to launch and Xbox was already an important platform for Splinter Cell, hence the Xbox 360 exclusivity at launch. The game performed pretty badly on the 360. The PC port was even worse with buggy lighting and crashes. The PS3 port that came later was also bad. My last playthrough was on the Series X. It enhances the game to run at 4K, locks the frame rate to 30 fps instead of dipping almost into single digits like on the 360. Double Agent, however, still stutters, even on such powerful hardware which is a shame.
All in all: bugs, strange changes to the lighting, bad performance and a flimsy double agent setting that misses the mark. It's an interesting project, but it's the worst Splinter Cell, because the earlier games were legitimately great, and later games, although different, functioned well and even provided players with an actually exciting experience, even if that experience wasn't quite Splinter Cell. DA is just meh.
Like I said, the year is 2006, Xbox 360 just launched, so there were still millions of gamers on the previous generation of consoles and, of course, on the Wii. Montreal would have to handle the other, last-gen version of the game. Read Part 2 here.