The exposure time is generally either too long or too difficult to control exactly. So you either run into reciprocity failure of the film (exposures >1s) or you have inaccuracies due to warmup/cooldown (different intensity and redder spectrum, both of which will have film-dependent effects on sensitivity) of the bulb consuming a significant fraction of the exposure duration (< 0.5s). A very bright tungsten bulb (bright enough that a 0.25s exposure is enough to do your sensitometry) with a mechanical shutter should be a very usable combination though.
Originally Posted by Rafal Lukawiecki
Tungsten bulbs also have a redder spectrum than daylight, to which film is less sensitive (in a very film- and source-dependent manner) so then you have to correct for that somehow. Strobes are a much neater and more-controllable solution.