As long as there aren't any levels as difficult at 6-2 in Ninja Gaiden I'll be fine. 
That's the truth of it really, there's nothing as tough or cheap as 6-2 and everything else you said about the difficulty is spot on. Not at all easy but very rarely is this game unfair.
I like the pace of the game too, it's very slow (part of this is because I'm playing it PALwise unfortunately) for the most part. The non-negotiable jumping means you have to be careful and calculating about it, especially with double jumps, otherwise you'll get killed. It's oddly satisfying having a slow paced platformer that demands mastery of the jumping mechanics. Somehow it's not particularly frustrating dying and going back to the start of a level. I think most of that is due to solid level design basically. The difficulty ramps up very gradually through the level (most obvious examples are the two onrails levels, the rafts one and the weird alien lifts stage) and there's that great sense of accomplishment when you finally make it to another checkpoint.
Helps as well as that the music is good, got a uniquely old fashioned feel to it, like a mixtape made for a Russian funeral.
On a final note, that level with the crazy lifts, it's so kooky with grapes on the ceiling and human hearts apparently sellotaped together on the floor. It felt quite out of place stylistically I think, and I'm glad after that it goes back to more typical castles and dungeons. There are enough bio-alien environment R-Type clones around that I was relieved they didn't carry the torch for that theme any further. Speaking of dumb shooter comparisons, the final final boss - bit Lords of Thunder anyone?