When the new difficulty first dropped, I'll admit I struggled hard. Ruthless (As a Bulwark main but I also had maxed Sniper and Tactical) was at times easy with randoms. Most knew enough to get by and I was very confident in my skills if needing to handle myself or carry some weight. 
The tethered armor mechanic is a bit... weird. I can't say I like it and it adds an unusual level of difficulty and punishment for being self reliant. Considering the very limited ways of getting your own contested health and armor, adding the requirement of being near players is annoying. Especially since people "stealing executions" happens, I for one like to engage Majoris 1v1 and use Banner for Contested health if I need it without risking someone else stealing it (only if necessary).
Pretext aside, Tethered Armor isn't actually that bad, unless in the last 4 hours they released a secret patch that made things easier, as long as you all stick together. Knowing how to get armor from Minoris quickly (Sprint attack into Sidearm Execution for one). Be aware, vigilant, and work with your squadmates and you should come out victorious.

For context, I just went through 6 out of the 7 missions (Skipping the first since I had that already) I joined a random pair of a Tactical and Heavy in the 2nd mission and we tore through every mission without a hitch. Baring a few downs, at times it was about as easy as when I first played all the missions on the easiest difficulty with two squadmates I had comms with. Now they may have had comms, but they more importantly had 'meta' loadouts of the Tactical Grenade Launcher and Assault Melta thrower which crushed Xeno and Heretic alike. 
Which leads me into the biggest thing: Do not go into Lethal without understanding the Meta. I can't speak for Assault or Vanguard's meta, but right now there is a clear Co-op 'meta' loadout. In my opinion, that's the previously mentioned Tactical and Heavy loadout, and a Contested Health Bulwark. Sniper can work for single-target damage, but the support function of Bulwark is unmatched. Lethal is not a difficulty to experiment, or level up equipment/perks. 

I have now 86 hours in SM2, 80 of which is most likely Co-op mostly spent on the first two missions. I failed most of my random Lethal matches, but once I matched with those two I had the easiest time of my life (more or less). Baring spotting equipment and enemies, only real communication that was shared was "I love you Jigsaw" after parrying a Lictor sneak attack on the last mission. I would have responded with a "<3" but for some reason you can't use "<" in SM2 chat (weird?). That aside, I may have been incredibly lucky to find two extremely talented players, but it clearly isn't too hard with how easily we walked through each mission. Knowing how to handle Extremis encounters and Terminus events is very important, but also managing the hordes of Minoris and groups of Majoris is the most important deal. Avoiding downs and health loss you can't immediately get back, plus keeping at least 1 pip of armor will lead you through any battle. And in Lethal, the only way to win, is together.