We only faced below issue in 1 particular Custom Form and that provided us ideas what we wanted to test / Validate and correct while we were doing transition Validation for New UCI in DEV

We found below 3 gaps which needed validation

1. Ribbon Button not showing in New UCI but visible in WebClient Legacy App
2. OOB Button visible in Legacy App but hidden or not showing up in New UCI ( there is 1 Display Rule: mscrm.hideonmorden)
3. Ribbon Button not working (RibbonAction.js has been deprecated)

So there are 2 things here: Any OOB Button on the entity is not visible on Form example: Other Activity  in Contact (Refer below screenshot)
OR
Any Custom Button on the entity is not visible, For custom button, we faced only in 1 form and that too was resolved opening Ribbon workbench and publishing it without any change

• FOR  OOB Button on the entity is not visible on Form example: Other Activity  in Contact (Refer below screenshot), we can follow below to enable but remember its not a recommended method and not mandatory that it will work.

There are many OOB Button that is disabled at UCI on purpose. So we can enable them only if required but I am not sure if that supported.

Click Here to check Post: How to add Button using Ribbon Work Bench

• Creating a new Solution with the entity (contact)
• Install Ribbon Workbench Solution and Open from Top Left Corner
• Select the Solution Created Previously with Entity

• Once Loaded, You can see all Hidden / Visible Buttons available on HOME , SUBGRID and FORM , We will select our Button “Other Activity”

• Right-click on Button and click on Customization Command

• In Display Rule, we will find a rule “mscrm.HideOnModern”, Deleting this will unhide this button on UCI,

• BUT THIS DOESN’T MEAN IT WILL WORK THE SAME WAY IT WAS WORKING IN LEGACY WEB CLIENT APP

If your button not working most probably you are using RibbonAction or Ribbon Workflow

• If you followed my old blog how to Trigger workflow or action from Ribbon Button and you are using this onto your ribbon button. so bad news is it has been Deprecated, below screenshot from ribbon workbench, you can go through it to remove it from Button

https://ribbonworkbench.uservoice.com/knowledgebase/articles/132235-create-a-workflow-short-cut-ribbon-button-no-code

• We can use JS to execute the same or I would recommend following this post: Execute Microsoft Power Automate from Ribbon using JavaScript – On-demand  Power automate flow using Javascript