I love mine. If your bag is working properly there may be the issue of different vinyl formulations and/or time 'in the bag' at work here.
Example: My original Janis Ian "Between The Lines" had a somewhat mild warp but along with that it was just uneven around the perimiter. ( I've flattened around 150 lps so far). I first put it in the bag for 4 hrs. It reduced the small noticable warp but the total unevenness was still there. I tried it again at 4.5 hrs. It came out basically the same. ( I've noticed that earlier pressings of lighter weight vinyl are harder to get flat). The point I'm trying to make is, that certain vinyl formulations seem to react differently when flattened with the Vinyl Flat. I believe this is addressed in the manual. I personally believe that the slow ramp up/down nature of the Vinyl Flat is critical to it's performance and protection of your vinyl.
I've tried the glass plates in the sun twice, and both times it ruined the lps. There is no way to simulate/control the proper temperature "ramp up/down" this way.
By the way, I've left lps in the bag on a couple of occasions, hours longer than recommended and they were not harmed at all.