Maybe I am the exception here, but my STI's, lowly but cared for Tiagra shift MILES better than my dt shiftered bike (which is also very well cared for btw).
Instantly accurate.
I don't have to take my hands off the bars, so don't reduce my control over the bike.
I can click through gears as I accelerate, instead of go, shift, go, shift, scrunch scrunch oops missed a gear!
I can shift gears constantly to keep my cadence in the sweet spot right near 100 so that my irritable knees don't get grumpy.
How are your d=DTs better than STI/Ergo and unless they are indexed, I don't see how they can be as accurate
Are you comparing like with like, does your other bike use the same mech and cassette and chain type ? There is something wrong with your downtube shifter if it isn't as good or better IYO.
As for taking hands of handlebars, well that's not relevant to smooth shifts. With linear changers I can go click, click, click, click instead of push, release, push release,push release, push release. ( am I there yet ?) I can use either hand to change gear front or rear. They last longer, believe me, even if you don't crash. Removing hands from bars isn't a bad thing, I do it all the time to signal, scratch my nose, drink, expel snot, it really isn't a problem. My handlebars are much more comfortable without STis, I found the hoods too fat and the levers too far away from the drop, they rattled. I can switch the indexing off if I suffer cable stretch or some other problem which makes it go out of sync. which has never happened