Here's my opinion: If you have a portable lamp that has a reflector, it could be used as a space heater that also lights up

But if you don't have a reflector you can use it as a space heater

Any light source is efficient as heater. Even >100 Lm/W systems turn only like 1/4..1/5th of the input power into light, so 3/4..4/5 goes into heat right away. Then, as the lighting lights up the room, most of it is eventually absorbed in stuff around and that turns into heat too....

Only difference is, that Incandescent (and Halogen) radiate more of the heat out as IR, while other light sources dissipate the heat at the lamp itself



But, we actually can be interested in how efficient Incandescent is as a light source, not as heater. The answer is not a blatant "very inefficient"

Incandescent is a lamp with very high CRI, but same as LED, it does not fill the visible spectrum evenly : While most colors can be rendered, many shades cannot. The region in which the light is lacking is in the Blue/Violet end of the visible spectrum. Colors and shades in the "warm" end of the visible spectrum are rendered well, this includes most human skin colors, wood tones and so on, generally, "natural" scenes

Incandescent light does allow visibility even at very low light levels. It is possible, that at some application an Incandescent lamp of some brightness will provide sufficient light, while other light source with similar Lm rating and even similar color temperature will appear too dim. In this case, a low power Incandescent could use little power just by being of low power rating, compared to anything else (more efficient for the same brightness, but less suitable) that could be used instead

Incandescent can be used where HID and FL are ruled out by their run up time, LED by light quality and so on, for example in stage lights (HID can be used only in a gobo system - which means that the light source is running all the time and masked when not in use, and LED falls short in providing the same ambience and light quality Incandescent does)

Incandescent light emitting center can be made very compact, and can be shaped in different shapes to get good efficiency when working with optics. For example, planar filaments used in many projection lamps, or the compact coil of LV Incandescents and Halogens. Incandescent floodlights have generally well shaped beams

Incandescents dim down well, and keep comfortable light spectrum through the entire dimming range down to zero

Where the illumination efficacy (so how little power is needed for a given visibility) is the top priority, I doubt any incandescent could even approach  the efficacy of LEDs or other sources. Mainly packages of 100lm and below: On these low levels theclassical sources just loose the efficacy too much (incandescent becomes way below 10lm/W, lower output packages even worse), while for LEDs tend to reach higher efficacies when the power goes down (easier heat management, small dies with less material blocking the output,...). At the sa,e time the LED cost scales down with the low power still quite significantly, the incandescents tends to be more expensive to make in lower wattages due to the things becoming too small to handle.
It is true an optimized color may give better visibility for the same lumens, but for that you may tune the LEDs to any part of the spectrum you want, even when the base remains a blue chip and phosphor (even e.g. 2000K would be more efficient even with some blue remaining not converted so wasted and not 100% efficient phosphor).

But not everywhere the efficacy is the real top priority. Sometimes it is heat resistance and there the incandescents have no competitor at all (you just can not create any other light source for e.g. a furnance interior, where the operating temperatures may go up to 1000degC and where you need some  permanent light for regular maintenance work - nothing could ever beat quartz non-halogen incandescent, when even a light pipe from some colder space is not an option).

Heating my "work area"

Heating my "work area"

It's not exactly incandescent, but I use my halogen work lamp for heating my bathroom.  It makes the small room feel like a sauna.  So nice on a cold morning

It's not exactly incandescent, but I use my halogen work lamp for heating my bathroom.  It makes the small room feel like a sauna.  So nice on a cold morning

This IS 100% an incandescent as well - the light is emitted as thermal radiation from a heated chunk of material...

Heating my "work area"

It's not exactly incandescent, but I use my halogen work lamp for heating my bathroom.  It makes the small room feel like a sauna.  So nice on a cold morning

I do this with my 160W SBMV lamps in my room at my father home, during the winter. The lamp heats the room only when at full regime. When the filament is overdriven and the arctube warming up, I don't feels its heat at all.