Hi! I'm joining as a generalist in a few months but I considered the expert track and the (separate, not to be confused) knowledge and research track as well. I spoke to over 30 people at various levels in both tracks that are relevant to my background and expertise. However, I clearly don't have the personal experience yet as my start date isn't for a while.
To clear up any confusion, there are three paths at McK with varying levels of specialization and client-facing exposure:
1) The classic path: Generalist Consultant Track (Biz Analyst -> Associate -> EM -> AP -> P)
2) A new, client services path: Expert Consultant Track (Analyst/Fellow -> Specialist -> Expert -> Expert AP -> P), which includes Solution Leaders (APs), Coaches/Implementaiton Coaches, and other positions if it's a client-facing solutions role/track
3) A "middle-office", partly support and partly client facing path: Knowledge and Research Track (various roles, including Analyst/Fellow, Specialist, Manager, and others), which includes R&D roles that are in developing and refining solutions like Product Owners
Expert is the equivalent seniority level to an EM on the expert track. I recently spoke to a former Expert Partner at McK who shared that 1) the speed of progression is more lenient with expert roles because you are building the expertise at each level, and the firm understands that takes time and 2) it's extremely tough to be an expert partner because the depth of specialization makes selling broadly difficult, given how focused you are on one area. He did say it is very rewarding though.
This means the track could take longer, but that doesn't mean it must be longer just because you're on that side. This also may mean that starting at the Expert level could be more forgiving than starting as a generalist EM, because of the slower up or out process that means you may not be shown the door as quickly if you're not able to adjust.
Ultimately, to progress to AP and above, according to this Expert Partner and the general process at McK, you must specialize and make it clear what you want to be known for, both inside and outside of the company. Because the expert track accelerates and focuses that specialization, it could be helpful for you to jump right into doing what you want (if that's healthcare).
I recommend speaking to people on both tracks and doing some research on LinkedIn to see for yourself which one matches your interest. Ultimately, I was applying as an advanced degree candidate and only had the opportunity to start as a generalist. However, I plan to specialize quickly and I've already built relationships with many people on the expert, solutions, and knowledge/research tracks. Happy to chat more if you're interested, but better to talk with people in the Healthcare practice directly. Good luck!