I have a thin merino wool jersey from Descente that's very soft and I can ride it in up to 90degF and be okay. Main issue is that the material is thin so rips easily and the seams come apart. It's been mended and I still like it a lot especially for races in the rain.

On the other hand I have a Woolistic merino jersey that's much thicker, heavier, hotter, and durable. BUT it's also relatively itchy and the sizes run slightly large. I've had luck shrinking it in the dryer, but it stretches back out especially if you keep anything in the rear pockets. I like to wear it when commuting in cooler weather (like it's been recently in New York).

Both jerseys stretch if you have anything in the rear pockets, to a point where it's flopping around.

If you can afford stuff like Rapha their merino/synthetic blend seems softer and less stretch-prone than the pure merino jerseys.