I mean, Izzy sang lead on four songs on UYI and ultimately he was credited as a writer on 12 songs, while Slash was only credited on 9.
The nature of the collaboration changed though. The way I always look at it, on Appetite you had five different musical backgrounds being melded together into a single cohesive style. It's like Queen (Axl), Aerosmith (Slash), The Rolling Stones (Izzy), The Clash (Duff) and KISS (Steven) were fused into a single band. On Use Your Illusion the band was five separate elements that never integrated, which led to a much wider variety of music but a less cohesive band. Slash and Izzy didn't have the push and pull chemistry on Illusion, instead they sort of broke down into a more traditional lead and rhythm dynamic.
According to Slash, the band's first three and a half years (from June 1985 to December 1988) was fueled by a mutual determination to make it at all costs. Writing the material, recording the album, going on tour, it was a single push where they lived, worked and partied together. Once they got off the road for the first time they found themselves without an immediate goal. They went their separate ways and basically disintegrated. They were only barely able to pull themselves together to do the Illusion albums (at the cost of Steven), and by that point there was no recapturing the lightning in a bottle. At that point Izzy, freshly sober, was very wary of the whole scene going on, while Axl's ballooning ego pushed Slash and Duff to binge drink just to tolerate him.
