The first thing to point out is that Federalism is not Unionism. It is not an 'alternative' to Unionism or Independence. It is a stepping stone to Separation.
The second thing to point out is that when we think of Federalism we tend to think in terms of an English Parliament.
However, it has become obvious over time that when the Labour Party in Scotland speaks about Federalism, they don't actually mean having a full-blown "English Parliament".
That's not really on their radar.
They only mean "having as many powers for Scotland as possible"; and whatever happens in England just isn't on their mind. Sometimes they call this "Home Rule".
For our part, as Unionists, there is little point in the SNP losing Holyrood in 2021, only for a Labour/Lib Dem Coalition to start advocating Federalism – or in this case, basically even more "Home Rule" powers for Holyrood!