My honest initial gut response was 'not merino'. I'm a fan but don't think it'd work as arm or leg warmers. Merino is good as a layer for warmth but less impressive as an outer for wind proofing (or you can read that as 'sod all use as a windproof layer' if you like)
I do have a pair of Rapha arm warmers (couldn't tell you the material blend) but can confirm that they are seemingly made for, achem, noodle armed cyclists
My leg warmers are Lusso, water resistant and a tad on the warm side. Arm warmers... can't think. I'll go and have a look in a mo!
Edit: they're Assos, I must have been feeling flush that week!
I guess though one really ought consider purpose and your own 'traits'. On the latter, I tend to run hot so keeping warm is not so much an issue than simply keeping the wind out - so my need is more for an outer barrier.
On the former, as a general rule, I kind of think of arm or leg warmers as a temporary measure. That is, you're going to be out for a while and anticipating the weather to change. For example, an early start and riding all day; stick the arm/leg warmers on to start and then remove them as the day warms up. Or, conversely, doing an overnight ride when you know temperatures will drop and you'll be in need. You can't tog up in full bibs and jacket because you'll overheat in the heat of the day, but you can stick on arm and leg warmers (and a gillet) when temperatures drop. I'm not saying that's there
only use (you can, of course, wear them whenever) but, for me, it's certainly where they come into their own.