The quality of the lens impacts directly the quality of the photos. How “good” a photo is (sharpness, color rendition, bokeh, resistance to flare and chromatic aberration, etc.), or how good it
should be, is very subjective. About any given photo, some people will say “It’s more than good enough for me” or “It’s too soft in the corners and there’s some distortion”... You get the gist. Therefore, it is quite difficult to advise someone on a question like “Should I buy this lens?” without having at least some idea about the level of image quality the concerned person is happy with.
In addition, there is the practicality factor: “Yes, I know this lens is not great but it’s so convenient and it does everything, and so when I am traveling, I’m willing to sacrifice some image quality for the ability to go with just one lens that never leaves the camera”.
All those viewpoints are fine, of course, but not knowing where you place your own personal cursor makes it more difficult to advise you.
So, I will use my own personal cursor and say this: think about how many prime lenses there are between 18mm and 300. Read up some reviews about those primes that are out there, designed by engineers who only had one focal length to worry about. Find out how many downsides and shortcomings can be found by serious reviewers in those products. Now, imagine those same optical engineers having to deal with the same issues they’re facing when designing prime lenses, but along an enormous range of different focal lengths... How good do you think the resulting zoom lens will be?
Now, you have part of the answer.
Then, look at the price. I don’t know about the US, but here in Europe you can still find that 18-300 zoom new for about 700 euros. Now, look at, say, the Z zoom lens covering 24 to 120mm: comparing apples to apples, you will find it new in Europe for around 1,000 euros. Why do you think a lens with a much narrower zoom range costs almost 50 percent more? It’s probably because the quality is significantly less (glass elements, coatings, internal mechanics, overall construction) in the 18-300.
Finally, look up the lens you’re thinking about on the internet: you will find plenty of reviews, many will be biased, some will have been made by incompetent click-baiters, but if you read up enough of them and use good common sense, you will be able to average out the various opinions and get a pretty accurate picture of that lens’s pros and cons.
Me, personally, I would stay away. Optics is a very difficult science, still largely analog and for which not much has changed for decades. Good glass is much better than it was 30 years ago, but it is still bulky, heavy and expensive. Professional primes fetch several thousands of dollars, and if they still sell at such prices, it’s probably for a good reason. Zooms are a great invention, but a lens that “does it all” doesn’t exist, and when it does, it doesn’t “do it all” very well. You will never see a professional with an 18-300, and very rarely a keen amateur...
But once again, we are back to the issue of perceived quality, which is subjective. With the F mount, I never went beyond a 3× zoom (the 24-70 or 70-210mm of the Holy Trinity). The Z mount being a game changer, I pushed my limits to 4× (the 100-400mm), and even, reluctantly, 5× (the 24-120mm). I would not advise to go beyond that, probably less with the more limited F mount. The optical issues that need to be dealt with are so vastly different when you design a super-wide angle of 18mm and a medium telephoto of 300mm that it is very hard to imagine an inexpensive lens doing both (and everything in-between!) well.
Still,
you may find that the photos it gives you are just fine, so at the end of this long rant, my advice would be to try it if you can before you buy it!