TB's rule of 'Electricity in = Heat out' is basically a sound one.
However some types heaters seem to be better than others in specific situations and there is at least a bit of science that shows:
(a) multi-finned oil-filled radiators are far more efficient than those with only a few fins ; and
(b) oil-free radiators (which are more expensive than oil-filled ones) are generally more efficient still.
https://www.dimplex.co.uk/mythbusters-oil-filled-vs-oil-free-radiators
Further, when comparing the use of such radiators to central heating, it has to be remembered that you're only heating what's inside the radiator, rather than what's also inside a boiler and lots of pipes.
I'm not a fan of central heating. (Indeed, if there was one thing that I could 'uninvent' it would probably be central heating!). To me, it seems to encourage people to heat areas of their homes that they're not actually using; I much prefer to just heat the room, or rooms, that I'm actually in.
I'm not alone in my view. My friend, who's the sort of guy who pays a financial adviser mega-bucks to manage his off-shore investments and who therefore isn't short of a bob or two, never turns his central heating on. Like me, he much prefers to only heat the parts of his home that he's actually using.
I've been using oil-filled radiators as the sole source of heating in my home for many years. If you do consider going down the same route, my advice is to forget cheaper brands and smaller heaters (with just a few fins), and to only use something like this (which is what I use myself):
https://www.machinemart.co.uk/p/clarke-2-5kw-black-oil-filled-radiator/
(It's also available in white)