I want to preface this with saying I've never had lots of experience with combo retaining champions, due to either their dupe reliance, no safe-nets, and requiring you to play with a tone of carefulness, with the AI being able to deny or screw you over in rampup phase.
But after getting a 1-2 6* Gem (which I highly debated using on Diablo) and using it on my 6* Guillotine 2099, and learning how to use her properly, my opinion changed.
Been running her in AQ alot lately, taking minis and main Bosses with clean solos (sometimes noded even).
And I totally get it know. I finally see the usefulness of combo retaining boost champions, with how impressed I've been with her performance.
Her damage is off the charts ramped up, and has decent safety nets, rewarding skill.
I can safely say G2099 has changed my opinion of her type of champion notion greatly.
Will highly look forward to using her in act 6 and forwards.