I mean what did people expect? The PSP can only do so much and has one joystick. Personally I'm enjoying Essentials, I would give it 7/10 but I'm giving it +1 for being Splinter-Cell-on-the-PSP.
Bad controls? Try Metal Gear Solid Peace Walker, now those are really awkward controls. Syphon Filter has better production values but it's more clunkier than Essentials as well but most importantly it lacks the personality and charm of Splinter Cell. I have tried playing Syphon Filter numerous times both on the PS2 and PSP and never could get into it.
The darkness is an issue in some missions yes, but stealth games shouldn't be played in daylight in the first place. If you're playing Essentials in dark you will be just fine.
This game simply makes me happy, I don't know what it is about it, I guess the spirit of the PS2 era and early to mid 2000s when games were still fresh and innovative (for me?). Nowadays tho just look at the Splinter Cell Blacklist, my god, how could the f up like that.