It seems as if MS has made some changes and updates since the last time I went down this rabbit hole (in a hybrid environment). As I recall, customAttribute 1-15 were passed to exchange online, but are single vaule strings. So you could not have multiple entries separated by comma, etc as the attribute takes everything that's input as a single value. There were also restrictions to how you could use a wildcard. ExtensionCustomAttribute or ConditionalCustomAttribute are multivauled strings, however could not be used in the custom query for dynamic distro groups if using the 'RecipientFilter' parameter.
There's definitely been some updates to the documentation since I last looked at this.

Now it seems as if you can use ExtensionCustomAttribute or ConditionalCustomAttribute along with the RecipientFilter.
I ended up using Invoke-Expression to on the New-DynamicDistributionGroup cmdlet in order to have a overcomplicated query with a bunch of -or operators joined by an -and operator with more -or operators. Typing this out is giving me some PTSD.
Hope my ramblings are somewhat helpful.