I would not class a Yanmar as well-built. I haven't run a tractor with them that I wouldn't consider just a glorified lawnmower. The engines I mentioned can be rebuilt from the block. A Yanmar cannot, and must be bored. Working on one requires metric tool sets. They don't have good torque or power at lower RPMs. It takes running them at full or close to full throttle to sniff the advertised HP, which on these tractors is often rated at the engine, not the PTO. Something like a Deutz will produce more torque and power at 1200 RPM's than a Yanmar at full-throttle.
I also don't care much for the tractor around them. These tractors were designed to handle only the power these engines produce. Bumping up into a John Deere with a PowerTech, a Challenger with a Caterpillar, or the like, these tractors were often built to serve certain size classes. With around 50 HP on the low end, you are getting a tractor, frame, transmission, cooling system, axles, etc., that were built to handle engines rated up to maybe 90 hp. Thus, the components on these tractors are almost "overbuilt", and can easily handle most of the stresses they would be put through.
John Deere's small tractors, and now I think Cub Cadet, are essentially rebranded Yanmars. I've run some of the Deere's, and I wouldn't give more than scrap value for one. They don't handle constant pull on the engine well. Plowing or shredding in the Texas heat, they get hot and run hot all the time. By 1:30 or 2:00pm, it's time to park them for the day. They do the same thing shredding when a few leaves or some grass and dust start catching in the radiator. They probably aren't bad for the weekend warrior, but they just aren't made to hold up to farm use. Twice, I've had the center pin pushed through the pin support and the rear end of the tractor.
These were all newer than my Massey Ferguson 235, but they weren't near the tractor. The only advantage they had was shifting. The clutch on the Ferguson feels like it takes 500 lbs of pressure to push in, and it's just really a standard transmission. I much prefer powershifts or shuttleshifts.