Thinking this quickly -thus not very profoundly- I feel Mejibray did have a good work there, mostly in the beginning of their career, like you say.
First, they were all very skilled musicians; and yes, that reaches Tsuzuku, too. I can agree on him having serious limitations to bring all his studio performance to his live performance, but the final result wasn't so different, mostly in their last era. On the other hand, he had a pretty strong and sensitive expression, in line with Kiyoharu's style, but also very unique. I mean, you could recognize Tsuzuku from every angle of the scene.
He wrote very beautiful lyrics, too, not over-the-top but kind of sophisticated in terms of poetic expression.
Second, they built-up very interesting and recognizable in-stage characters, which is the key of the visual aestehtics of 'visual-kei' in difference with other approaches to 'glam' in rock music. The best example of this was Meto's wheelchair and creepy teddy bear. Paradoxically, this something that the majority of "vk" bands can't achieve, mostly because they basically do rip-off certain famous characters of the past; I think we can all agree that Meto, Kouichi and Tsuzuku looks are still blatantly plagiarized to date.
Third, they did had a very unique sound. The tones Mia, Meto and Kouchi's chose for their instruments created that 'sandy' sound that, when coupled with Tsuzuku's shrieky vocals, made the band instantly recognizable, no matter which song you could listen to. And in terms of songwriting, they did have some memmorable songs. I can think of "Raven" and "Decadence"; certainly there wasn't something like that in the scene in the past.
For me, the weak point of Meji was the obvious plagious they did to some hit songs from classic bands (Deluhi, or even the Gazette), and also the 'formulaic' basic songs that they started to put out during the second half time as a band, more-less after "Theatrical Blue Black". Plus, I've never found their B-sides so exciting either. I think they got over-produced, and thus over-exploited. Maybe so much that they ran out of ideas, so they had nothing else to do rather than making 'catchy' songs for bangya to do furitsuke, copy-pasting their previous clichés in different ways, while choosing to put on the most eccentric attire and make the sexiest posing on stage. So they obviously gradually lost all their music value and started to become an 'idol' band.