>>455297Half of a fabric's suitability depends on how it's woven and used in clothing, and there are a number of companies now that make merino "tech fabrics" which are breathable, warm, and sewn so they cling to your body just right. In that aspect, merino is a legitimate option.
Comparing pure merino shirts to pure polyester shirts, the merino often comes up being less durable, which, IMO, is the major downside. The merino clothes that I own have tended to develop small holes that open up quickly, but don't progress past a certain point (usually, they start out dime-sized, and never get any bigger).
In general, the advantages of pure merino clothing are that it tends to breathe a little better than polyester for being able to keep you warmer for its weight/thickness, it can insulate while wet (but not as much as while dry), it doesn't chill your back (as much) when you remove a sweaty pack from your body, it gets smelly from body odor slower (but who cares on the trail?), and it subjectively feels better against the skin than polyester (merino is not itchy to most people, although some with more sensitive skin have reported itchiness).
Merino-polyester blended socks like Smartwool or many other brands (I've had good luck with Darn Tough) have the added advantage of durability, since the polyester fibers are used in areas that are subject to higher wear, like on the heel, and ball of the foot. In my experience, Smartwool style socks and pure polyester hiking socks perform similarly as far as doing what a sock is supposed to do - keeping your feet dry and free of blisters - but the merino blends do a little better in cold weather because they can keep your feet a little warmer.