Somewhere along the way I got the impression that a watch strap must be made with veg tan leather, that chrome tan wouldn't be sufficiently robust. Having dug in a bit more to the details I now see manufacturers are using some chrome tan leathers for watch straps (Chevre, Saffiano and others) but I'm wondering are there still certain types to avoid for this application?
I've always felt that most watch straps were too thick and stiff and never had the luxurious feel of nice leather gloves, for example. So I'm definitely looking for options on the softer end of the spectrum. If we consider a 3-layer, cut edge strap with a liner (something like zermat) and reinforcement (velodon or ripstop) and the outer layer, is there anything you really shouldn't use for that outer layer?
Most commercial watch straps will be chrome tanned leather. The majority of hobby leather crafters will probably use veg as it's easier to get hold of, simpler to work with, and removes the guesswork that you are experiencing.
Not all chrome leathers are created equal, some are quite stiff and stretch resistant, whereas others are soft, stretchy and spongy.
I'd recommend getting hold of samples from a supplier before ordering an entire hide or skin. You'll know almost instantly whether or not it's suitable for watch straps just by the density and hand feel.