Short description of error
When using the filmic view transform, and setting the sequencer transform to srgb, linear, or filmic log, the image output by the vse will be significantly lower contrast than it should be. Similar issues apply when using filmic log view transform.
In fact, there appears to be no combination of settings that will allow the use of filmic or filmic log and allowing the sequencer to use filmic color transforms while still outputting at full brightness.
While it is possible to make individual strips display properly by setting each one to 'raw' color space, strips such as the color effect strip do not have this option, and will always be much dimmer than they should be.
Exact steps for others to reproduce the error
Switch the scene view transform to Filmic.
Add a color strip to the sequencer.
Set the color strip to full white color.
Here is the solid white strip in filmic mode, notice the waveform shows the brightness at about 75%.
here's what it looks like in standard mode, brightness is at 100%.
And a .blend file for easy testing