Well, in the end it all really depends. I think when you are trying to improve your line work, you should put a lot of effort into each line, simplifying and ghosting each line, taking your time etc, as for some applications your use of line can be a big deal, especially if the lineart will be prominent in the end result (like inked comics for example). It also helps when sketching and doing gestures to improve your line quality and confidence.
That said, you can totally "hide" bad line quality, you can be as sketchy, chaotic and messy as you want to be before pouring buckets of values or colors in your preferred rendering method, essentially vanishing any remnants of line art if that is how you work, in fact, some artists paint directly, entirely foregoing lineart for the most part. That's not to say it's right or wrong, but when learning I think it's best to do your best at any given thing you do, avoid being sloppy at all times, and make each mark meaningful and as easy to read as possible, both to do the exercise of understanding what you're doing at a pace your brain can actually comprehend and also to constantly reinforce concepts you are learning
It's hard to say what's most challenging. It's so subjective for me, but I remember struggling hard to do gestures when starting out
cheers
