A lot of people like to say it's either "too easy" or "Super lethal". Can't there be a middle ground?
Yes, there certainly can be a middle ground, but probably not in the way you're thinking?
Like anything good, it is a balancing game. You certainly don't want every every battle a struggle to the bitter end, nor do you want every challenge to seem like a cakewalk.
Anyone else find this to be true most of the time?
Yes. All the time. As others have said, the published adventures are mostly designed for newer players and DMs, meant to offer a "fun adventure" more so than a "challenge" to more serious (?) or dedicated (?) or experienced (?) (pick your term or supply your own!) players.
If something is so easy, from skill checks, to saves, to combat, why even roll? Just save time.
In general, 5E rewards specialization with *near-*automatic success. In truth, it is far closer to 65% typically than 80%, but with even some minor boosts or advantage, 80% is very attainable!
So, what happens is, you have a "perception PC", with maybe
alert feat, high WIS, proficiency or even double proficiency in perception. Someone with
darkvision as well, probably. This is the PC that typically ruins most chances for surprise, hidden traps or secret doors, etc.
Then you have the "investigator PC", the character that--again through high ability score, proficiency (or expertise via a feature or feat), advantages, or whatever pretty much can overcome most locks, non-perception "detections".
You might have the "know-it-all PC", who's got all the best INT skills with high bonuses.
The "sneak PC" (often the perception PC) who can bypass any guards, etc.
The list goes on an on. PCs with high skill bonuses, advantage due to magic or elsewhere, make most ability DCs pointless.
Move on to the damage soakers, the always-win-initiatives, the high-damage dealers, and on and on. Add
magic into the mix, disgustingly simple in 5E, and just compounds the situation to make it worse (or better, depending on what you want out of the game...).
I remember in my first 5E game, a player (part-time DM) had a PC who was the ultimate skill-monkey. Proficient in every skill and had "expertise" or double proficiency bonus in something like 10 or 11 of the 18 skills. Once she got Reliable Talent, forget any DC under 25, especially when should could give herself advantage on any check...
Obviously, with combat you can be just as dominant. Social challenges, explorations, are all fairly pointless if you rely on the die roll to make them challenging or dangerous. Such characters steamroll over published WotC adventures.
So, as you know, your options are to do the things you can do as DM. Beef up the encounters, make your own adventures, or house-rule the heck out of it to try (nearly always fruitlessly) to balance the numbers for rolling.
And while you do this, you look for that balance I spoke of in the beginning. Trying to have a blend of challenges, where the "key PC" isn't always the one who gets to make it easy. For example, you separate the PCs, so only one group has the "perception PC", maybe the other the "sneak PC", etc. then run the two groups independently for a session or two, until they (hopefully) finally make it back together.