I'm posting as a reader here...
I can see why worldbuilding advised you to post here. Worldbuilding tends to want well-bounded questions, not the more woolly "meta-questions" like this. That said, the answer does come down to worldbuilding.
Building a believable world is hard, and most authors fail at it to some degree. Successful series get the opportunity to retcon those mistakes - for example, JK Rowling introducing Apparating and Floo Powder as the major ways for wizards to travel, retconning away the problem of how you would get all those people through the Leaky Cauldron entrance to Diagon Alley. But there still has to be enough internal consistency for suspension of disbelief.
High fantasy tends to live in medieval or pre-medieval worlds. Of course this ties in nicely with all the old myths and legends from around the world. But crucially, this is pre-industrial, pre-scientific and pre-scientific-method. By removing this potential crossover, the author has to address a whole lot less questions about how those crossovers work. To stay with the holes in JK Rowling's worldbuilding, HPMOR famously deconstructs how a kid with a reasonable interest in science could trivially combine basic magic and basic science to become almost godlike.
Rowling as a writer is very good at plot, and keeps things moving fast enough that you mostly don't have enough time to stop and notice the holes. But when you do have time to stop and think, you realise just how badly assembled that world is. So that sets your standard - you need to be writing to Harry Potter levels of competence before your worldbuilding can withstand Harry Potter levels of incompetence.
The problem you run into of course is that wherever you are, your entire society is shaped by its technology. Roman day-to-day living was shaped by its ability to get water around efficiently. The trading and colonisation empire of the Vikings (yes, they mostly didn't rape and pillage) was based around their shipbuilding. Medieval towns grew up along navigable rivers or coasts, because that was the best means of transportation; Victorian towns by contrast first grew up along canals, and then later along railways, and by the late 1800s we had regular commuter villages around major cities with people travelling to work by train just as they do today. Transport enabled the commuter lifestyle. So if your technology allows magical transportation between places, you need a reason why people would live in cities. (Answer usually: they don't.) If your technology allows magical creation of food, you need a reason why people would cook themselves. (Answer usually: either the Harry Potter solution that you can't magically create anything from nothing, or the Star Trek solution that replicators need some kind of feedstock which you might run out of.)
And then there's how your new tech/magic affects the plots you have on offer. With phones today we're never permanently out of contact with people, and we always know where we are, and we always have a map, and we always have a torch, and we always have a recording device. There have been many books and films created which hinged on the protagonists having or lacking one of those. All of those are now historical artifacts of a time that has passed, as much as stories of Beowulf or Achilles. You can look back at fantasy written in the past, where wizards had magical maps to show where they were, and wizards could create light from their staff, and then again the party couldn't get lost because the wizard knew where they were. (But take away the wizard, or take away the phone, and what happens...?)
With one degree of freedom, it's easier to answer those questions. With more degrees of freedom, it's harder. The most obvious question to answer is: where does the energy come from for magic? because if you can move something or heat something by magic, without a source for that kinetic or thermal energy, then you have a perpetual-motion machine. And the consequence of that isn't just "you can do anything", it's also the eventual heat death of the universe. You'd better have answers.
Sure it can be done. Terry Pratchett progressively introduced technology into Discworld, moving through the printing press, the telegraph and the steam engine. Scott Lynch has a Renaissance-like world which includes magic and alchemy. Charles Stross has one setting where maths is magic (and hence so is computer science); and another setting where it's possible to walk between alternate worlds. Tad Williams' War of the Flowers created a fairy world which is a warped-mirror version of our own with magic. All of them spent a lot of time making sure that their worlds fit together both technologically and sociologically. And in the roleplaying arena, D&D has included varying amounts of technology, and Shadowrun has rolled magic into sci-fi; in both those cases the challenge for the game creators has been ensuring a playability balance between magic and tech.
And in all this, it's worth bearing in mind Clarke's Third Law. What matters is the outcome, not the mechanism. We may pretend that Star Trek is hard sci-fi, but really it's just space opera with better scenery. Magical food creators, magic wands to shoot each other, magic spells (anything Scotty or LaForge comes up with), monsters with magical powers, humans interacting with dark elves (Romulans), orcs (Klingons) and halflings (Ferengi) - it's all there. Whether it's magic or technology only matters if it's on a scale we can easily follow, and beyond that it doesn't really matter whether you say it's nanotechnology or that it happened by magic. (Neal Stephenson's The Diamond Age, for example.)
If you can make your worldbuilding hold up, then great. The problem is that it's hard to break out of the "traditional" tropes and still make it stand up. That's why they're traditional, because they work. It's a safe place to start.
And yet, if you stick to the traditional tropes, it's hard to have something new to say. No-one wants yet another rewrite of LotR - Terry Brooks just about got away with it with Sword of Shannara by being the first, but everyone since then has needed some new story to tell, or something new to add to the basic quest idea. That doesn't necessarily need a new setting either; Stan Nicholls took traditional fantasy and wrote it from the point of view of the orcs. Even Game of Thrones is entirely derivative in setting and concept; the originality comes from adding the question of whether the humans will stop killing each other for long enough to fight the common existential enemy.