I've shot 777 in my Knight LK-93 (AKA Wolverine) for probably 20 years with no issues. I use two 50 grain pellets and a 300 grain Hornady SST sabot. My aim point is the same at 50 yards and 100 yards.
I converted my Knight to use 209 primers. I typically use the Winchester ML primers, but have used other brands over the years. No ignition problems, well, only one. It had been pouring so I give it a pass.
I've also got boxes of pellets that have been open several years and I've never had to ditch any for moisture. They are always stored in plastic ammo cans but sometimes inside, and sometimes in the garage.
I've heard lots of good things about BH 209, but with my stash of 777, accuracy, success on deer, and reliability of ignition, I can't see myself changing anytime soon.
I will add that after about 5-6 shots, it is stiffer loading the bullet, but I haven't noticed any difference in performance. I am a clean freak when it comes to my ML so after every range session, it gets cleaned. If I hunt and don't shoot it, it still gets a light cleaning.