Speaking as someone who has never played a game of soccer in his life
Would fullback be one of the hardest positions to play nowadays? There's a lot of expectation to get forward and create as well as defend. Not to heap excuses on him, and I'd rather have someone who defends first and attacks second, but perhaps like WC2022 suggests, we should temper expectations.
It's not as hard as central midfield. You have to attack and defend but the ability to run repeatedly is pretty easy, so long as you can spend the time getting fit and have serviceable speed. Midfield should be box-to-box and you have to be wary of everything around you and think quickly as there often isn't an easy out option.
The job defensively for an FB is generally to channel players wide where you will stop them putting in an effective cross. If they run in-field that's where your DM/s should step in, because if you follow them, you create space for the opposition's overlap. DMs provide insurance in the same way as you hope your CBs can stop attackers from scoring if you apply pressure to the crosser.
FBs also have the advantage of the line being nearby, limiting the area and unpredictability of the area they have to manage and giving them an easy out for kicking the ball when under too much pressure or simply wanting to slow things down/re-structure. Going forward, you also have the advantage of the line being at your side, so you know an opposition player isn't about to come at you from your blind side. You can also often observe the whole field from your corner of it and consequently choose to cut infield, run the line, rake a pass, etc with relative confidence. For those reasons, it's a great place to blood youth.
However, the key factor in all this, is knowing when to run forward/overlap/jockey/dive-in/etc. It's an instinctive role and stuffing up leaves your side exposed on their flank and often leaking goals. So it's easy to look bad; and it's easy to look great in one aspect while failing all the others. As it stands, a lot of our FBs have a particular strength (Smith's crossing, JoGo's defense, Moreno's speed) and so it is a 'hard' role for them to impress in all areas. But Can is in a similar conundrum. He doesn't have one stand-out strength (other than his looks) and potentially could master all areas. Central MFs or CBs can save them, but if not the wrath will fall upon you for not doing your job. Hence why this thread is so manic-depressive about him.
And that's without getting into the unique challenge of playing up forward where the punishment for error isn't as immediate, but your time on the ball is limited and your immediate opposition freely belt the ball into touch...