If we think what goes in must come out, then as long as no energy goes up a flue or is in some other way removed from the house, they are 100% efficient, so a tungsten lamp is 100% efficient as all energy in comes out into the home both lighting and heating the home.
However we don't look at it that way, we consider a lamp is to produce light, so any heat is waste, even in the heart of winter.
So a heater can heat using conduction, convection and radiation simple school education, other than using a heat pump, what goes in must come out, however not quite that simple, if the heater can make us feel warm without heating the air as hot, then air changes in the house will lose less energy, so it is a case of combining the convection and radiation in a way that allows us to heat the air less yet maintain control.
Inferred has a problem in switch off and one feels cold and switch on we feel hot so switching it on/off in the normal way does not work, so we want a fast acting semi-conductor switch which alters the mark/space ratio to adjust the output of an inferred heater, and we also want to combine it with convection so a heater can be made to make us feel comfortable, using a lower air temperature so is more efficient.
So if you normally heat the air to 20°C but by using a heater which combines inferred with convection you feel as warm at 18°C and the outside temperature is zero then there is clearly a saving, simple maths 20/18*100 = 111% efficient or if you say can't have over 100% then non inferred is only 90% efficient, in real terms not as simple as that as heat loss in the home is not linear, but in real terms a radiator black in colour is likely to give off inferred and white in colour unlikely, but it would also need a thermostat which was using the mark/space ratio and switching at around once a second, and any mechanical thermostat would not last long doing that.
So there is some truth in the claim that one heater is more efficient to another, however not sure if you could measure it, and you would need to be in line of sight to the heater, so if we consider the three bar electric fire with the quartz tubes that glow red, they have the ability to be more efficient to a oil filled radiator, but only if we can control them so they switch bars on/off automatic to maintain the lower air temperature and we sit so in view of the bars, plus curtains are closed of course as inferred can pass out of windows as we know when we feel the suns heat in the other direction.
I honestly feel the chance of controlling the inferred to really gain money due to using less power is slight, so even though possible, and can't say adverts are untruthful, I think it hardly matter what heater you get. To be more efficient one has to have a bench mark, so if we say the bench mark is the heat pump and that is 100% then the panel heater is maybe 92% efficient but if the panel heater is the bench mark then panel heater is 100% efficient.
So if we look at a 58W fluorescent tube, with a HF ballast and compare to a LED with a switch mode driver than the LED is maybe 5% more efficient over 10.000 hours life, but three times the price, so more costly to use, so money wise fluorescent still wins. But compare to tungsten it's very different when used in open air, but when used indoors then heat has also to be considered.
Simple fact, if from outside your home you can see when the lights are on, you must be wasting energy, you want all the light energy to turn to heat before it leaves the house not after, so black curtains in winter, white in summer.
