uBO/PB make you much easier to fingerprint uniquely due to not loading advertising content on all web requests, which could be considered harmful in a scenario where you don't want to be tracked. Facebook Containers extension offers the same fingerprint as "doesn't have a Facebook account and uses a browser that blocks tracking by default" outside of the container, which is far less unique these days than it used to be.

Note that this doesn't diminish the effectiveness of uBO/PB at blocking requests — but I think most of their users don't realize how much easier it is to identify who uBO/PB users are, relative to the rest of the world on less obvious setups.