This is more of a comment than a question.
We recently deployed and then reverted dynamic format strings on hundreds of measures in a large Power BI model on a P2 capacity. We discovered 2 issues that forced the reversion:
1. The formatting worked great in Power BI, but Excel users connecting to the model experienced timeouts and over-utilization of memory on larger queries (queries that worked without issue hundreds of times before). The same report structure that failed in Excel would return almost immediately in a Power BI matrix. I assume this is due to Excel quering in MDX, which probably doesn't handle the formatting efficiently (the Excel group really needs to dump MDX ASAP, we run into issues all the time). However, I couldn't confirm this due to issue 2.
2. Traces in DAX Studio or SQL Profiler no longer returned any results for queries using a dynamic format string measure. This made it very difficult to diagnose issues as they were occurring.
(Edit) A third issue that I forgot to mention is that, when attempting to connect to the model from SSMS and process a table/partition, an error would be thrown: "The database compatibility level of 1601 is below the minimal compatability level of 2147483647 needed for <DYNAMIC FORMAT STRING MEASURE NAME> in table <TABLE NAME>.FormatStringDefinition at PowerBI mode. (Microsoft.AnalysisServices.Tabular)
