In comments on the (relatively!) recent post on monarchy in SF/F I remarked on balance of technology as a key element of successful worldbuilding. Regular readers will not be surprised that the subject has come up before, explicitly so in Space Warfare XV: Further Reflections on Laserstars. (Three years ago - yikes!)
The particular example I gave there of 'unbalanced' technology came from a science fiction novel I once had, in which figured a World War II era heavy cruiser that had been refitted with smoothbore guns.*
This is something you probably do not want - unless you do want it, perhaps because (as in the book) you are dealing with a post-apocalyptic setting having a mix of surviving high-tech artifacts and the much more rudimentary technology that the survivors can contrive on their own.
Needless to say, 'unbalanced' technology has its own interest and appeal, which is one of the factors that has made the post-apocalyptic subgenre so popular. And, to get really pedantic - something this blog never fears to do - unbalanced technologies also have their own logical balance.
My personal guess, for example, is that people who could maintain a steam turbine power plant in operable condition would also be able to keep precision-machined guns in working order, and even provide ammunition and powder, if perhaps in limited supplies.
Likelier 'unbalanced' technologies might include sailing vessels with auxiliary motors available for limited use, and a limited, perhaps dwindling supply of modern weapons. Or drones with canvas wings on aluminum-tube frames, powered by lawn mower engines and controlled by smartphones.
What is not particularly likely, I think, is a simpler linear regress, such as steampunk-era ironclads (cool though they are). Building a 10,000 ton ironclad requires the ability to harness resources on a large scale, and a massive if unsophisticated industrial base. And anyone who has those things can systematically research more advanced technologies, especially if even a few artifacts have survived for reverse engineering.
Disclaimer and proviso: Of course the rule of cool trumps all these considerations, which is why post-apocalyptic futures tend to be heavy on punk rockers and motorcycles.
Which brings us back to balanced technologies. Alas, having brought you this far, I really don't have magic solutions to offer for keeping a futuristic setting's technologies in balance. The further you get from a souped-up present day, the less obvious it is what technologies should fit together neatly.
Does FTL imply that you 'should' also have torch drives for normal space operation? Or normal-space propulsion as demi-magical as FTL itself is? Or could a future starfaring civilization (linked on general principles) be using two-stage expendable rockets to get into orbit, and rendezvous with upper stages to get to wherever the jump points are?
In this case there are plausible (at least plausible-sounding!) arguments going both ways. On the one hand, FTL implies a revolution in fundamental physics, which ought to enable a whole range of new technologies involving pretty much every aspect of life. On the other hand, a century after Einstein, Newton still provides a pretty complete explanation of how our actual space propulsion technology works.
What I can say - unhelpful though it may be - that the general principle of balance of technology also applies to other aspects of the speculative subgenres of Romance. Other things equal, magic too should be in some kind of balance: If every hedge witch can work powerful spells (even if not always reliably), it is hard to then turn around and have magic effects be few and far between.
A bit more vaguely, I feel that this principle applies to social and political worldbuilding as well - suggesting that it is a bit problematic to project agrarian-age institutions such as feudalism or the Roman Empire into a post-industrial future.
On a more cheerful note - from a writer's perspective, not necessarily that of your characters - the balance of technology is dynamic, not static. No small part of the entertainment value of the 16th century is that its technology was in rapid transition, as indeed was its broader culture. Early-modern tropes, such as royal musketeers, existed alongside medieval tropes such as knights in full armor.
And in an encounter it is not always a given that the more 'modern' combination would prevail. The same is broadly true in any era of rapid technological and social change.
* Does this story element ring a bell with anyone? As I recall, the book (which probably dates back to the 70s, at least) also had an attempt to launch a space ark, which did not end well.
The image, from a site called damngeeky.com, is described as a post-apocalyptic PC case mode. To me it looks more like a steam powered laser. And no one should be surprised that most Google Images under 'post-apocalyptic technology' involve punk rockers, motorcycles, or both.