Just finished my fourth season with a Wolftooth elliptical on my XC bike with flat pedals. I have a circular chainring on my trail bike (and, of course, my road bikes).
The first two seasons I waffled back and forth on the elliptical benefits but, after finishing the third and this latest season, I give the elliptical a solid “meh”….it is neither beneficial nor detrimental, overall.
I have not noticed any benefit with climbing with the elliptical. I have found the motion of the elliptical “soothing” on long flat trails but that’s more a psychological than physical plus.
Technical rocky or rooty sections are a bit of a challenge with an elliptical ring in it interferes with effective pedal-ratchet due to the “dead spot” you mentioned. I’ve mostly gotten used to it and developed the muscle memory to work around it, but it does catch me off-guard now and then.
After seeing the folks I ride with come to the trail with pie-plate-sized 50 tooth rear clusters on their rigs I’ve concluded that good old-fashioned gearing is the only real solution to making climbs “easier.” ; )