Most optical pedals especially the Morley's can be major tone suckers imho...... of course the pedal steel guys rely on their volume pedals constantly so maybe there's someone out there with an informed opinion (from a pedal steeler's perspective).
The major plus of the ernie ball is that they use a higher end and , most importantly, sealed clarostat pot. In contrast, the boss uses a cheapo light duty mini pot. There are a couple of reasons why a regular Vox/Cry Baby casing isn't the best volume pedal. The rack and pinion gear doesn't allow for a complete rotation of the pot (which is fine for a wah circuit) but awful in a volume pedal. As well, most of the pots are not closed so it's isn't long before dust and dirt makes the pot scratchy.
Optical pedals do not have these problems however some of them have a slight delay and ...the buffer will affect the tone and/or get clipped by any boosters in front of it.
Andy