By 'pressure', I presume you mean stress.
There is a potential for stress caused by any type of screen enclosure, due to the fact that the cham doesn't necessarily have an unobstructed view of what is going on outside the enclosure. It is possible for someone to walk up to the enclosure without the cham noticing them until their hands are already inside the cage grabbing for stuff (like when their food gets changed). This undoubtably does stress the cham out because it takes them by surprise.
If you've ever watched a cham in outdoors in a tree, where it has an almost unobstructed view for quite a long distance, you would observe that it spots potential intruders a long time before they see it. As soon as the cham detects someone walking towards it (even if they are still many metres away), it will manoeuvre itself so that it becomes almost invisible to the intruder. And that's also why they have those 360 degree rotating eye turrets - they don't like being surprised
So in a dark screen enclosure where it doesn't always have the ability to detect intruders from far away it might get stressed out by not knowing where potential threats are.
Having said that though, some people feel that certain chams - particularly females - prefer a screen enclosure because it provides an extra layer of 'security', fulfilling almost the same role as foliage in allowing the cham to feel more hidden.
Like I said in my earlier post, many keepers have kept chams successfully in flexariums/reptariums. I don't think you need to worry too much about stress, provided there isn't too much traffic around the cage, and that whenever you are near the cage you move slowly and deliberately so as not to startle the chameleon.