Every time this argument pops up, I end up wondering if it’s really about architectural complexity, or if it’s about sticking with the old and known.
Now to try and answer your question, I think a relational database is a lot more complex than a simple key value store, but if it's a good implementation, it will hide this complexity from the user, and using the database will be rather simple. It can be as simple as a key value store since you can just use a relational database like one. Note that this hiding of complexity is by design, it is one of the main goals of the relational model, it's not accidental.
Example Go's mgo driver by Gustavo Niemeyer was simple and effective, but abandoned. The official drivers is unnecessarily complicated. And Go's need to have "context" everywhere adds to this, but MongoDB is not complex. Idk I'm not some super genius and picking up MongoDB was really easy. Querying (aka aggregation pipelines), you have to think of that as "pipes in bash". It's in the words aggregation pipelines. find | groupby | sort | filter | select. Something like that. It's not SQL it's different, but not complex, sorry.
Like, if you were never exposed to rxjs and are now trying to build things with it doing
$stream.pipe(
switchmap(),
filter(),
etc(),
)
stream.map().filter().etc()
Now write DBA scale ISO SQL 2016 / Transact SQL / PL/SQL in that interesting Mongo flavoured language, including database engine debugger integration, and JIT compilation.