The downside is that with all their code and plugins they're bloaty and slow (about 15% slower than my non-WP sites running on the same server). Make of that what you will.
Properly run CMS is really all about cache to make hardcoded HTML instead of all the bell and whistles along with all the plugins (that get generated each time user visits).
The "page generation" time is what slows the server responses and create the majority of the wait time. Just simple cache plugin should suffice for the majority of webmaster without needing custom codes. Properly powered servers can really help with the generation time as well.
The "code bloat" arises is really just a tiny speck in comparison to some of the ad units / redirection calls / or even videos that get served. 10kb of CMS HTML bloat vs a 1 MB video ad? I ain't going to spend many hours more for that extra 0.1% of bloat.
In the business, it definitely helps to know HTML, CSS, and even PHP. Utilizing old school editors may help with that learning process. But I think going back to work on production sites via HTML editor is backwards (at least for sites that are already on CMS). Sometimes it's also the ease of ability to be able to update sites forward to address future changes. CMS does that perfectly with minimal coding knowledge.
I am a little younger in my 30's and I did use editors back in school on hobby sites before I entered the "real world". I always thought it was a huge pain in the ass. I am happy how tech has advanced this past 10 years. WP has really matured along the way too with so many stronger and powerful plugins that get things done.
I am seeing higher RPM this past couple of days! Pretty happy about that.
[edited by: martinibuster at 3:25 pm (utc) on Jul 24, 2017]
[edit reason] Added quote to give context to this response. [/edit]