I think they are great for preventing vermin but my personal view having watched them used is that they can create ore anxiety and agression around meal times. Having watched demos on youtube they also seem to allow dominant birds to guard the food source.
I use two round hoppers as I find that everyone can get round them at the same time and the lower ranking birds can use the hopper as a shield and eat out of site.
With the training Ghdp you are aiming to encourage the birds to make a conection between aproaching the feeder via the plate in exchange for a reward initially. so for the first stage the food reward dosent have to come from the hopper. If your birds trust you, you could feed them with a bit of cheese from your had. They use sight and smell to find food but also learn to respond to sounds. So if the food is not visible and the smell isnt strong enough then they wont be that keen to get to it.
There are lots of ways that you could encourage them but it does depend on your birds and the relationship that you have with them. Calling them to the hopper and slowly opening it for them to eat and closing it when they have eaten, opening it again etc. using cheese, corn, grapes, worms or anything else that they go crazy for will make them much more interested in exploring it than covered up pellets.
If its the guillotine style clatter of the dropping lid that put them off then they are not thick...they are sensible. perhaps you can prop it so it dosent fully close and then their is always a tantalizing gimps of food to encourage them.
Chickens also learn from watching you and each other so if you have a confident, friendly bird train her and she will teach the others. Leaving the lid up teaches them that the tray contains food, when you shut it they will revisit and peck around hungrily. Eventually they will stand on the plate and the food will be exposed but this is as a result a much longer process.
My birds free range so if getting the food was too difficult they would leave it and eat grass instead. If they got hungry enough they would all come and find me and pwork for corn. Thats what they do when the turkeys snaffle all the pellets. But if I introduce something new I always do it with treats and they pick it up very quickly.