There's two truths in this problem.
If you have average gear that has a true line input, use it.
BUT better gear, and any recent gear that follows the electronics handbook, has a mic input that is also the line input. That's because these mic preamps have such a wide gain range, that you can use them as a line input, simply by reducing the gain.
The only thing that might not be ideal, is the impedance. But the days of impedance matching have long gone. Your typical mic preamp has a 2 to 5 kOhm impedance at the input. A "true" line imput should be 10 to 100 kOhm. But modern gear doesn't care, so it really is a moot point.
If you're running real vintage tube stuff, it becomes a real problem, technically. But that's just the kind of stuff they do, those who own the real vintage stuff. To add colour, warmth, depth, or whatever they like to hear in harmonic distortion.
A way to check in the specs if the gear you're eying works well with a combined line/mic input, is to check the overall gain. Anything below 60 dB gain will be mediocre. Anything above should work.
If we take the example of an RME interface, it has 80 dB overall gain. In theory, the output would produce 10 V from a 1 mV input at full gain. Of course, the output can do 10 V, but the following input might not be able to take that without a bit of distortion. It's just spare gain, for those mics that don't even reach 1 mV. At 0 dB gain, there's a number that's not often in the specs. It's the minimal gain. In this example, it's 20 dB. Eg, a 1V at the input still results in 10V at the output.
It's also why good gear has a headroom of at least 20 dB over 0 dB;
Where do you encounter the problem? Any battery fed recorders. Except stuff like Sound Devices etc.
Because the mic inputs run on low voltage, low current, the design isn't by the book. And these usually distort a lot a 1V, 0 dB input. The better ones have line inputs, unfortunately often it's just a -40 dB pad, which adds a litlle noise.
There are a real 32 bit interfaces. >15.000 €, sans options. These don't even need to adjust gain. Their range is so big it doesn't matter if the signal is 1V or 1 mV.
To us, mere mortals with a mortgage, RME with it's digital gain is the closest we can get to 32 bit heaven. It's only 24 bit, but with a true digital gain. 0,5 dB is precisely 0,5 dB. Every time, not dependent on mains power or moon phase.