By that do you mean they take less physical pressure to trigger the switch? Or that the plunger is loose inside the barrel?
When I think about it, I would prefer a heavier(?) action. Not that I particularly enjoy the incessant and loud clicking of Happ buttons, but a little resistance isn't a bad thing. And I definitely do NOT want a loose feeling button (that's what those Yenox make me think of.) Primarily I just want the relative thickness of the whole stick to be as slim as possible.
Sounds like I'm leaning toward the Seimitsu... ever had the Seimitsu buttons fail on you?
As Franco said, the Sanwa's take less pressure and thus feel like theres less actuating depth (maybe there is as well? I dont know for sure), though that being said, if the Sanwa's feel like a NSX, the Seimitsu's are a very respectable RX-8, and happs are more like an F-150.
I built two sticks with the Seimitsu's for me and a friend, both around the time that Soul Calibur 4 came out. Since then he and I have played SC4, ST:HDR, and SF4 pretty heavily, and neither one has had a problem. In seeing both sanwa's and seimitsu's, I would say they are of similar build quality, and both should last as long as you expect a push button should last (read: forever).
The Yenox, 1 out of 6 had some plastic parts break off (pretty much my fault) but are definitely a lower quality button, not the same grade as Sanwa, Seimitsu, or Happs. Since I was using them for buttons that I didnt press or look at very often though, they work fine, and I wanted the shorter depth over a happ to fit everything inside my boxes.
Edit: None of the buttons feel loose, at least to me, sanwa, seimitsu, happ or yenox. I mean they will all rock a little if you rest your finger on the plunger and move your hand laterally. If theres a scale of 1-10 with 10 being the hardest you have to press to get a button to register, Id put a sanwa as a 2, a seimitsu at a 3, a happ at 7 and a yenox at 9. The yenox just feels more plasticy and hollow when you pound the button, and the spring is definitely stiffer.