Well, the tube problem seems to require specific material deformation calculations for stretch, etc. which vanilla SketchUp doesn't do. It isn't material properties oriented.
That doesn't disqualify it as a CAD. There are and have been hundreds of CADs which don't do material specific deformations -- and those that do have sometimes required expensive add-ons or versions to do it.
Even then, SketchUp is extendable and it is possible someone has written material deformation plug-ins for it. I don't know, it isn't what I use a CAD for so I'm not familiar with what is available.
I have designed and built many wooden boats, and even built an ultralight plane, wood and canvas type, and pretty familiar with traditional lofting. I don't know why your plane specifically had dimensional problems. To know that would require knowing how you arrived at each shape specifically step by step in SU. For instance what was your template and precision setting when you drew it? Within that precision did you create components in a way that produced cumulative errors, etc. Did you use any Plug-ins, like Bezier, etc.?
I'm fairly certain I could draw a fuselage in SketchUp with components accurate to a considerably higher tolerance than 1/8", but it would take an understanding of why yours didn't work. I was trying to get to that by asking you about a simple procedural exercise in my last post. That wasn't answered.
It is impossible to troubleshoot a software issue without getting down to specifics of user steps to reproduce the issue. I know. I've worked in software technical support. Generally you try to simplify the case to see what is going on. Putting up a full rendering and saying it is off by 1/8" has no specific solution or cause. Procedural errors can arise in any CAD or program.
But beyond that, I'm not trying to convince you to use SketchUp, since you already have Rhino and AutoCAD which work for your specific needs and you are happy with them. Why would you want to use SketchUp it at this point?
On the other hand, there are a huge number of people who could (and do) use free SketchUp as a CAD, and get perfectly usable tolerances for the specific need.
The Engineering template has an available precision setting of "0.000" although that must be selected by the user over the default. If a person needs higher precision, free SketchUp isn't the tool to use. Get out your wallet.
On the other hand, could be you just don't use SketchUp to its capability. If you don't set the precision to a high level and happen to be in the Architectural template with a 1/64th tolerance, then yes, cumulative procedural errors can quickly get you a fit-up of + or - 1/8th. That shouldn't be a surprise. A real life tape measure graduated in 64th used to cut multiple pieces of wood laid end to end will quickly yield errors of that magnitude.
We're running pretty far afield of the original topic, which was simply an answer to the OP whether anyone uses SketchUp as a CAD, with CNC. Or the Phlatboyz G-code generator plugin as a CAM.
My answer is, yes, I do. It works for me. Lots of other people do, and I see no reason why others might not want to try it for a majority of what people do.
Users do need to learn it, and its specific methods (like any CAD), and how to avoid procedural inaccuracies, how to set it up for maximum precision, or add plugins to extend its functions. But it's free, and....
whaddya want, egg in yer beer?