I’ve got a couple of pairs of the Morvelo liners and they’re okay for me. For perspective, on the road or cross bike I tend to wear good quality bibs – Rapha or similar – and the Morvelo liners aren’t anywhere near as good. Then again, on the mountain bike you move around on the saddle a lot more and change position generally, so my take is that they don’t have to be as good.
Also… in hot weather, a lot of bibs use a lot of non-mesh, high-elastane fabrics above the waist – the bib bit in other words – which in turn reduces breathability / wicking on the torso.
I’d say the Morvelo ones are pretty decent for the price. I find the leg grippers stay in place. The chamois is okay in a value sort of way. They’re maybe a little long in the leg. And while the fabric’s less sweaty than full-bore Lycra stuff, I reckon they’ve missed a trick by not sticking some mesh panels in the outer leg to improve airiness when worn under baggies in hot conditions. I’ve never noticed the flatlock stitching to be honest, I’ll got and have a look at them in a minute.
Anther option I use are DHB Aeron Shorts, the waist ones, which are fine under baggies and have a better chamois than most liner shorts including the Morvelo ones, and still leave the torso with maximum breathability. But then they cost about twice as much.
Have also used Mavic liners in the past with lots of mesh, but the chamois was a bit useless ime. Just too thin and poor fit. I think the Morvelo ones are okay and great value if you can get them for £20 still. HTH…