I have worked in then managed a textiles applications laboratory for 10 years, then worked in the technical marketing of engineering plastics for 30 years.
I have a degree in textiles technology, which while not an engineering degree, involved quite a bit of engineering, along with chemistry, physical chemistry and polymer chemistry. I do have an engineering apptitude, and physics was the subject where I never had to bother studying to much to get a good pass.
In the technical marketing role, I worked for a number of years as a "Market Development Engineer".
In this time I have been involved with a very large number of projects, many in the automotive and the appliance industries with companies like GM, Ford, Mitsubishi, Bosh, VDO, Toyota, Electrolux, Black & Decker, Sunbeam, Bendix, Borg warner, Osram, GE, Ramset, Pandrol, just to mention a few.
Many of these involved both qualified and unqualified engineers.
I have seen many good and many bad decisions by both qualified and non qualified engineers.
The main difference between the good and bad decisions was experience in the type of materials used and the inteligence, overal broad experience (formal education is just one form of experience) and intuition of the engineer involved.
Another major factor is the environment in which they work. Once management overrules engineering and decides to go to production "anyway", despite sturctural problems, or when stylist have the last say, and insist on makeing an unsound structure "because it looks nice", a disaster is iminent.
On the other hand, if we wait for the engineering to be "perfect" or we market a sound but ugly product, a disaster is also iminent.
To calculate stresses in complex , I think an engineering degree is a big help
To come up with effective designs, I think apptitude has a lot more to do with it than a degree, but the degree can't hurt.
I think to get a degree you need to be a good scolar, and apptitude helps, but is not essential.
To get a job in engineering without a degree, you need a track record that proves apptitude, and some terserary education helps a lot, but university education is not essential.
Like all things it comes down to a sensible ballance and good projects come from team effort where all the necessary skills are present and recognised and respected by all.
Regards
pat