I judge difficulty based on how often PCs drop to death's door and/or killed. It's not a set ratio of drops per combat but an impressionable feeling. There are obvious ratios. If a PC drops every combat it's hard but not necessarily too hard. It's a warning sign where circumstances matter. If it's two or more PCs it's too hard. If you're losing count how often a PC is killed it's not even a game any more. BBEG fights don't count in terms of dropping. If no PC drops great, but a PC or more than one PC dropping is not unusual. If a PC is killed every BBEG fight that's still a problem. Those are the extremes. Lesser ratios are still hard until some subjective point is reached where the difficulty is just right the risk of dropping makes victory sweet, but if it does happen you recover and it's all part of the fun. PCs deaths are memorable because it's rare.

That's the DM side of it presuming general player competency and no ill will intention meant about the DM though "killer DMs" who enjoy PC drops and deaths do exist. On the player side new players do earn their own personal experience points so to speak learning how to play so eventually it becomes easier. At some point they're no longer new players and have general competency. However, there are players who can't or won't learn thus the game is always too hard for them. I knew a 5E paladin player who hardly ever went into melee and instead insisted on using a crossbow. When he deigned to go into melee he would not smite unless another player told him to. D&D is not hard, but it could be hard for a particular player. It's not the game for them.