Regarding why basic harness is more comfortable, it is a little counter intuitive at first. Think about under water, not on land. If you dive in tropics with no exposure suit, your rig should be more or less neutral to begin with. The basic harness does nothing more than secure the rig on your back. There shouldn't be much weight on the harness at all. If you dive in cold water, where you rig is negative, but you are also in thick suit, then the suit is for padding. But even that, your rig shouldn't weight much in water. Next time you dive your transplate, feel your shoulder strap once you get settle down under water (neutrally buoyant and in trim), you should straps are not tightly test on your shoulder. In fact, it is the whole reason why BC has a chest strap. Without that, your should strap may just slide off. In other words, the shoulder pads do nothing in water. Now, if you have to hike 2 miles in your rig to get to your dive site, yes, shoulder padding is helpful
The 2nd reason is the fit. With basic harness, proper adjustment is comfortable. With basic harness, once adjusted , things stay. and since the webbing is quite soft in water, it confines to you body. It won't slide off. That is why you don't see chest strap with basic harness. With transplate, the fit will be different every time you mess with the quick release/adjustment. Sure you can lock down the adjustment with tri glides, but then all you are getting is easier don/doff. Proper fit is comfort, not additional padding.
The 3rd reason is specific to transplate's poorly design shoulder pads. They are very stiff and quite wide, not comfortable at all. A example of good shoulder pads are the ones in Halcyon Infiniti. Very soft and comfy.
The last reason for me to leave transplate for good is the number of d-rings. Too many aren't necessary good. During a dive, you really can't look at the d-ring then clip things there. You go by muscle memory for clipping, and by feel of your hand for unclip. With the kind of implementation in transplate, you are very likely reach a different d-ring than the one you intended to use. There are really 3 on each shoulder. Two intended d-rings and one for holding the quick release buckle/chest strap. Just too many. One on each shoulder is just right. You always know you reach the right d-ring.
Basic harness are not without its shortcomings. The major one is getting in and out of it if you wear thick suit. It really takes some get used to. Then it is the comfort level on land for long period of hike/walk, which I have no intention to do.