I noticed that it was significantly harder last night. It took me 3-4 attempts before a group got in with me that could handle it. Do the modifiers change every day? The reason it was so hard last night was because of the "glass" something modifier that gives you half shields and health but much faster recovery. I started doing better when I adapted to that: frequently ducking in and out for quick shots and then finding cover to recover shields. I'm pretty sure I didn't have to do that the first day and it seemed much easier then. I also remember the elements cycling every several seconds the first night - I assumed you'd have a boost whenever your preferred element came along. Last night was just void burn.
I've gotten 8 or 9 Gambit Prime armor pieces (between the Sentry and Reaper classes) and not a single one has a great roll yet. Given that they need to be upgraded I'm really waiting on a perfect set. I am almost about to upgrade my mote container though. It worries me that then I will be a in a much harder version of Reckoning (but one that I think will give me +3 gear instead of +1 gear?). Also, the point tiers are 3, 6, 10, and 15. So you really need some high level gear to get the top perks.
I also went off to do daily/weekly stuff I hadn't been doing very much of recently to get some Powerful gear and Prime engrams. It's really the Prime engrams that you want. I've gotten those as high as 658. My Warlock is at 654 now. Linkin, I saw plenty of people in Reckoning at just above 640. It doesn't have a power level requirement - you just have to have the motes to wager (which you need to get from Gambit Prime). But I'd guess that the 640's would have a harder time with it, especially with yesterday's modifiers.