I'd happily build them for non-commercial one offs, but as already mentioned you then lose a lot of the cost advantage.
A careful build will take at least 8 hours intense build time (and probably an awful lot more), plus the time spent getting the parts and prep. The headline prices for these kits don't often include things like hookup wire, terminal blocks, heat shrink, solder, shipping, extra necessary tools etc etc. They can add a lot to the final price.
Skill-wise I don't think you can confidently do most of them as a very first project, you need good soldering skills and basic electronic knowledge straight away (it's a lot of money if you get it wrong). It's never a completely straightforward build, and if you break something you need to be able to follow the schematic in order to work out a way around it. Most builds start with wiring up the power section, something you definitely don't want to be doing wrong.