If you rely on the work of others (machining) and do your own assembly, your chances for success actually improve. The catch is, you have to invest in inspection tools to check the machine work and a good torque wrench. Some find the assembly of a motor daunting, but there are so many resources on the web that anyone with a mechanical aptitude can do it.
You have to be able to check the machine shop's work if you build it yourself. You need inspection tools like a micrometer and Vernier calipers plus snap gauges to check bore, rod bearing and main bearing inside diameters. You can't skimp on the tools as they have to be as good as the ones the shop is using if you're checking their work.
The car I currently own originally had "pro-built" motor ('70's funny car driver turned speed shop owner). It had a history of spun rod bearings and rocker to retainer clearance issues and came to me as a basket case motor along with the car.
I dry fitted the existing rods and pistons and found the block had been excessively decked, the rods were all different lengths and the piston domes (Venolias) on the longest rods were hitting the cylinder heads (722 heads) when hot causing rod bearing failures. Of course the valve geometry was all wrong from the excessive decking. The builder ground notches in the beams of the Harland & Sharps rockers to clear the retainers.
There is no excuse for a pro builder to turn out a motor like that.
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