This is a strong point. Nobody likes a rapidly flashing light (extra annoying, seems broken, can be hard to differentiate from "on but dim"). Then again if you flash slowly then you'll have some appreiable amount of time when it's off and that can include the entire opportunity you have to look at something.

Ideally (for me) you could have smooth high-low alternation or colour alternation.

(I recognize that something that looks like emergency services, e.g. alternating blue/white may be illegal, and that colour-blindness may limit this approach.)