If you encrypt at a block level then the pre-authentication parsing code is relatively simple. If you encrypt on top of a filesystem, it's more complicated and relatively likely to have vulnerabilities (seen in practice in iOS at least). It may not be that important, but it's a factor.
Sure, but it's also the case that encrypting at the block level means that upper-layer filesystem code needs to be prepared to deal with corrupted metadata, which is stored in encrypted sectors that attackers can randomize.
