ok, since i have apparently not explained enough (or at all) here's what that meant.
"badly performed": miyavi still isn't that great of a singer, better than your average vk-dude but still, he's especially not very good at writing/singing nice vocal melodies. also while he is arguably one of the most fluent english speakers in the j-rock corner, the engrish is still strong with him, so singing in english really doesn't help much, as does the fact that each time he speaks/sings in english he somehow feels the urge for either pseudo-inspirational nonsense or utter bro-talk (yo yo, imma rawk teh wurrrld, murtherfurkers)
"questionably produced": the main problem here is this one. all the critic above was always there (apart from the english singing which started later), but so far his albums have offered enough other stuff to keep oneself entertained plenty. don't know much about his duelequartz/VK phase, but later with kavaki boys and 7 samurais there was a huge band involved that all did interesting stuff at a top level. on "whatsmyname" he could really show off his pretty unique guitarslapping and looping stuff, whilst having a (in jrock-corner) pretty unusual garagy 60's production sound to it, and a sick ass drummer. this was a guitar-focused album (where he's good at) with added vocals. in terms of vocals he additionally was also able to cover up most "melodie" and engrish problems by simply adding a little scream/shout value to it (it's kind of a cheap trick, but it works...)
and now in the production the obvious main-focus lies on the vocals... i find that to be a bad choice.
dance pop synthie stuff is totally my thing, but it's just not well executed here. maybe because kpop has ruined me in this corner but here it is really basic and boring stuff. i would understand switching the groovemonster that is bobo to an electronic dance beat if it was an ok dance track, but it isn't. so this is just another reduction of quality too (yeah, he plays in the videos and live, but certainly not in most of the cd-tracks).
that leads to another problem, miyavi isn't that well of a songwriter, he always had a knack for good hooks, riffs and otherwise interesting and cool song-elements, but to stretch that over a whole song most of the time didn't work that great, but as mentioned earlier that didn't matter much, because there was stuff to keep you entertained (weird ass band with step dancer / manic guitarwanking+distorted drums)
in both those areas miyavi isn't really bad, but not strong enough to carry an album with it alone, when the other parts strip away or are badly executed.