According to FAQ on migration a migration done in this way (if users flag it as off topic/belongs on another site) is rejected if none of the tags on the exists on the target site. Quote from What is migration and how does it work?:
A migration can be automatically rejected before it even gets migrated if any of ... The question does not contain any tags that exist on the destination site, with three exceptions:
(There are also some other situations when migration is rejected, but I only included the one which is tag related.)
This means that to migrate questions such that none of the tags exists on Physics.SE, you would need to involve moderators even if the migration path is open. So I think that if a migration path is discussed, it should be also checked whether questions to be migrated are likely to have common tags.
Although more important criterion is actually whether there are enough questions being migrated.
EDIT: As pointed out by Asaf Karagila (physics) is an intrinsic tag on physics.SE. This means that if a question being migrated has physics tag before migration, the migration will not be rejected.
I have noticed a few tags which are common for both sites. (I am not sure whether there is an easy way to compile a complete list. I found this ones by looking at the most popular tags on Physics.SE and also on looking on questions that were mentioned in discussions about migration to Physics.SE here on meta.) Here are some such tags: quantum-mechanics, classical-mechanics, quantum-field-theory, general-relativity, special-relativity, fluid-dynamics, fourier-transform, notation, terminology, percolation, data-analysis.
There are also some tags which are rather close. (Such as mp.mathematical-physics here and (mathematical-physics) on Physics.SE, other examples are differential-geometry, algebraic-geometry, algebraic-topology) In such cases, creating a synonym on Physics.SE could help against migration getting rejected.
I have tried to get all MathOverflow tags and Physics tags from Data Explorer and then I simply used grep to get the intersection of the two files.
Unless I made a mistake, the complete list of common tags consist of the following 56 tags: linear-algebra,