The ability to only assign one Permission Set to a user, is very limiting. If I have one user who needs access to a feature, but I don’t want to give that to everyone from that team, I have to clone their existing permission set, add the new feature, and assign that to the user. We have just started using HubSpot, and the number of Permission Sets we need is getting out of control.
My suggestion is to have a Main Permission Set, and then allow Addtional Permission Sets to be assigned to a user.
Salesforce does this with every user having one Profile, and then allowing multiple Permission Sets.
Note, I know that i can have individual Permissions per user, but that is very unscalable and would be a nightmare to manage.
Additionally it makes it impossible to use SCIM for access control.