Depending on field, there can be substantial downsides, because academic papers and PhD theses are fundamentally different beasts.
There are certain things that people just don't publish in academic journals, but that is nevertheless useful information. Experimental or theoretical failures is the obvious one that everyone knows about - it's much harder to publish an article about "I tried this and it didn't work", than "I tried this and discovered something cool". Further, journal papers (which are not called "letters for nothing") often have stringent word, page, figure, and even citation number limits, forcing academics to pick and chose only the most critical information to include. That excluded information is often to be found in PhD theses, where those limits don't apply. If it isn't, then it's lost forever, pretty much.
Speaking for my own field (experimental atomic physics), I go looking for academic papers when I want to study advanced scientific topics - because there is nowhere else to get most of that information - but when it comes to technical details, long-form PhD theses are second only to informal, personal discussions with other people doing nearly the exact same thing as you.
"Technical details" is, itself, quite a loose term, but for me that has included things like:
- Specific part numbers - "Part X from company Y achieves the best performance of all 18 options we tried"
- Unexpected failure modes -"this type of glass suffers from thermal lensing, so you have to have windows made from this other type of glass"
- Undocumented specifications - "Part A from company B is specified to do P, but can also do Q"
- Unknown suppliers - "samples from "
- Techniques that didn't work and so were not published
- Techniques that did work, but weren't published - you'd be surprised how many of these there are.
- Alternative solutions to expensive hardware - "you could buy the all-singing-all-dancing computer controlled widget from company X (that you don't have the budget for), or you can build your own with an Arduino, this code, and half a day's soldering"
- Simple solutions to problems that nevertheless would take a considerable amount of time and error to identify yourself - "you need a 4th order high-pass filter for this signal because a 2nd order can't solve interactions with this other effect"
Off the top of my head, my own thesis cites something like 40 other PhD theses for useful information that saved me time, energy, and my supervisor's funding. That was out of 250-300 references in total, so 15%; the vast majority of the rest would have been journal papers.
Nobody likes writing a PhD thesis - and it's true, very few people will ever read them compared to, say, Lord of the Rings or Harry Potter - but it is a valuable source of information which never makes it into published papers. Think about all the things that you have learned over the course of your PhD - if you never write it down, you may as well have never learned them.