Can you use bicycle boulevards for effective interval training?
In principle yes, but keep in mind that, while you will be using public places you will be likely meet other cyclists who are not there to train and will act, to your eyes, just like moving obstacles: kids riding and suddenly stopping or turning, flocks of teens looking at their phones while riding one hand and then suddenly stopping their bike across the lane, elderly people just taking a ride to enjoy the sun, etc. 

You, while busy in training, might even call them like CSO (Cyclist Shaped Object), but in reality they are as legitimate users as you are. And I know fellow amateur cyclists who prefer to practice peloton techniques on the highway rather than on a cycling lane (dumb, isn't it?).

With that in mind, how heavily will that affect your training schedule? If you are really chasing the numbers and the performance I assume the impact will be heavy, and maybe it is better if you look for a better place (either less used or dedicated to training). If you just want to put some more effort, go for it and take it as a good chance to also improve your reflexes.