I'm gonna suck WL4's dick some more because I feel it has actally a pretty good balance. I didn't find the bosses particularly hard; what fucked me up was that I knew that my ending would suffer if I did badly, and I was too anxious to waste my frog coins on failed attempts, so there was a shit ton of pain and excitement involved in those fights because I wouldn't rest until I got good. ...I know that was just my own way of playing it, but the point is that what makes the game hard is that it encourages you to take on the challenge if you want to actually feel fulfilled at the ending. The levels were fairly easy if you played them straightforwards, but if the bosses gave you too much trouble you would eventually have to go back to get coins, which was really entertaining if you explored and solved the puzzle rooms: again, good balance, since you have a way to work around things but nothing is as fulfilling as gettin gud and ACE DAT SHIT. (I think Shake It had trouble relating both aspects; the levels and bosses are really fun, but getting treasures and completing missions felt more like optional sidequest that didn't grant you any in-game reward -as far as I'm concerned-, so you're most likely to play it straight-forwards without weighing down what it means to fail)
WL1 has a similar case: the levels themselves aren't so bad, but exploring them to get the treasure and make your ending worth it is hell. Having to pay up for gameovers with your treasure was pretty damn harsh and made the bosses all the more nerve wrecking, but in the end its like... the stakes is what made them so hard. An easy boss can be real hard when you can lose so much if you don't beat it properly, an easy level can have much more value it serves as a build-up to a boss who holds the weight of the actual progression in the game. Both work super well HNGGG
I personally prefer the latter out of that sense of build-up, I feel it's more climatic <3 unless you get the shit ending in which case wario ends up homeless and its all your fault protect bara bby 2k16
I'm not the biggest fan of of WL3's structure, but one thing that I love about it is that the bosses aren't in predictable places. They just show up out of the blue on any chest of any level, and I think that's the biggest sense of mystery in 3 other than what parts of the map will spring into levels. For the actual fights themselves I prefer WL4's tests of endurance and speed over the one-hit-failures in most cases, but the actual structure of where they are is cryptic but not unintuitive, much like WL1's treasures.
Difficult levels and somewhat easier bosses. This because, and I'll use my experience of Wario Land 3 while talking about this, the levels were really fun and challenging and perfectly build around the controls. Now while Wario Land 4 also was really well build around it's controls, I feel that those levels felt more different from each other, all having some kind of gimmick. Trust me, I love those levels, but there just was something really nice about how you could fit together all the levels in the old games.
Now as for the bosses, I have a very good reason as to why I think these are better in the old games (or at least for Wario Land 3.) Basically, I feel the bosses in Wario Land 4 were very... generic-ish bosses actually. Not as in design, but more as in how you beat them. It was just bash, bash, bash. In Wario Land 3, bosses were VERY varied. You had things like Football(/soccer) and a thing that fell under water when you hit him and you had to beat him whilst climbing. Basically all of them were different, and, again, made very good use of the games mechanics.
