Depends on the partner and the PMs
The success or failure of these projects comes down to three things: choice or partner, effective partner project management and effect client project management.
If the partner is DXC, for example, they are screwed. But any other D365 partner should be more than capable.
Project management is where these projects succeed or fail. If the client PM doesn't understand the business (ie. GW just hired a PM or hasn't done a full requirements document with complete gap analysis), the chances of success plummet because of all the moving parts within a company and the PM's ability to align the company. If the partner PM is weak (ie. they can't say "no" at appropriate moments to defer a new pet requirement to post-go live), then the project will collapse. If the partner PM can't marshal the necessary development and testing consultants, and ensure those consultants understand the company, the project is doomed.
Yes, ERP transitions and big and complex, the opportunity for failure at every turn. Good project management from the client and partner can make even the sketchiest situation succeed.