Ah, racks. If canister warmers are the hare, rack warmers are the turtle. They take forever to heat up, and when done the towels were pockmarked with cold spots and weren't that hot. At the 7- and 9-minute marks, when the canister warmers had grilled their towels well-done, the towels on the racks were still cool. I kept checking as they slowly warmed up, and after 75 minutes I thought I'd given them ample chance to impress me. Max towel temperatures didn't come close to the canister warmers. No surprise, since rack and towel are exposed to room-temperature air, and the towels heated unevenly because only parts of them are touching the bars. It helped to weave the towels between bars before I turned the racks on, but I still ended up with cold spots.
The Conair has a timed delay, where you can set it to start heating 6, 8, or 10 hours later, so you can set it before bed and have it start heating towels shortly before you wake up, mitigating the slow heat-up time. But it performed even worse than the Amba at heating up towels. My end result with the Conair was a cold towel with a ruler-sized strip of warm, but not hot, fabric in the middle.
The Amba doesn't have a timed delay, so you'll have to wake up early just to turn it on, then go wait around for a while before you hop in the shower. It's not something I had time for when I had to get to work.
Major drawback number two is that, unlike canister warmers that have plastic outer shells to shield you from the heating elements, racks are just chunks of hot metal hanging out there for you to bump into. The Amba gets DAMN HOT. I shouted a lot of four-letter words into the air from brushing past it accidentally. If there's a towel completely over it, no big deal. But if you contact bare metal, you're going to start singing soprano. The Conair didn't get as hot. Instead of fear, I had mild annoyances when trying to slide past and avoid the bars. It wouldn't exactly be pleasant if I touched the metal with bare skin, but I also wouldn't jump through the ceiling.
Canister warmers are the way to go. You can drag them into the living room to heat up blankets, robes, and sweatshirts on a cold night, and they're safer, quicker, and make for hotter towels.