Often when I write these, I tend to get too pro-MFT for others and even myself sometimes, even if my intention is to promote the use of MFT format.
I will try my hardest here to apply my usually analytical brain to the facts of the matter to help you make up you own mind. There will be personal thoughts at the end, but not biased I hope.
The sensor.
The MFT sensor is about one quarter the size of a full frame sensor and about seventy percent the size of an APSC crop sensor. This does not mean it is only 25% as capable, because the law of diminishing returns says that more is usually less more as it goes. In the past it was always up to the bigger format to prove it’s worth, not the smaller one and smaller formats have generally punched more efficiently.
The reality is, software and processing often makes as much difference.
The sensor doubles the magnification of a given focal length, which means long reach can be achieved with smaller and cheaper lenses that fall closer to “ideal” design parameters.
Depth of field is also increased simply because the shorter lenses render it. A 300mm lens on MFT format will have the depth of field of a full frame 300mm, but the equivalent reach f a 600mm.
Your own needs will dictate what is important here, but usually when working for a client, more depth of field is a benefit, so if the widest practical aperture needed is for low light, there is a two stop equivalency gain compared to full frame (f1.8 on MFT = f2.8 full frame at the same effective magnification).
Wide angle lenses are correspondingly very short focal lengths increasing design stress, but given the difficulty of making good, sharp edge to edge wide angle lenses for larger formats, this seems to even out. My experience with MFT lenses is soft edges are rarely a thing, something I was always aware of in full frame
The sensor is squarer, which makes lens design easier also, or more to the point the perfect circle can be smaller as width and height are closer to the same.
The sensor will produce, when compared to the bigger format, either less resolution or more noise pixel to pixel. The sweet spot seems around 20mp for MFT, enough to produce bill-board sized work (assuming you are looking at proper a bill-board viewing distance). The pixel density of 20mp is roughly the same as the many of higher MP sensors in full frame formats.
These are often cited as ideal for cropping in on, so in some ways MFT is the high res full frame sensor cropped.
With modern processing applied, if a natural looking, sharp and clean image is needed and a little more time is allowed, clean ISO 12,800 images are possible, but realistically 6400 is the safe maximum if premium quality matters.
Lower res full frame sensors (24mp or under) are capable of several stops more clean output, some with dual ISO processors for an added edge. The two stops of extra depth of field and double the reach for the focal length of MFT can offer effectively two free ISO stops in some circumstances, but the “ISO free” feeling of a full frame dual ISO sensor cannot be matched.
This means that in real terms, a slow lens on a some full frame cameras can match a fast lens on an MFT cams at roughly the same resolution and with similar depth of field. The size, price and weight advantage of MFT can often be matched in these circumstances, but only at the same resolution.
This is basically the full frame makers applying the math in reverse for much the same effect.
Like a Renaissance painting, a shot taken with an ailing 16mp EM10 mk2 (the screen connection is twitchy, so I use it as an EVF only studio cam), a cheap 45 f1.8 (at 2.8) against my Manfrotto Walnut backdrop, probably worth more than the cam or lens these days and a cheap brolly/flash combo (at minimum power). One of MFT’s random advantages is a boost in flash power as I can select the “ideal” studio aperture (f2.8 in mft), which is a couple of stops faster than in full frame (f5.6), it bites you though when in bright sun and you want shallow depth.
No sharpening or other contrast processing in C1. Worth mentioning here I guess, I have two full frame cams that I could have used.
Full frame sensors are generally better at mistake mitigation.
Poor exposure, white balance and colour shifts can generally be bought back better (from my experience), but personally, I have not compared like to like. My own real life testing, done using older MFT vs new full frame cams has proven out the Goldie Locks rule;
If you muck up a little, either can be saved.
If you muck up a lot, neither can be saved.
If you muck up just right, full frame is sometimes better.
The long running argument has been that full frame is called that because it is the “true” base sensor size. This is based on 35mm film being the original work horse size, which it was not professionally even then, it was just the most convenient. The fact is 35mm film was only selected because it was a handy convenience, being the film industry standard, but the 3:2 ratio was not loved for publication, considered by many to be a useless size.
The true format for film makers is a loose group of sizes lumped into the “Super-35” category and it probably should have been for stills shooters as well (the rare half frame movement), which is 35mm film shot sideways across the length, how it was meant to be. This is roughly APS-C or even close to “Academy” MFT in size..
MFT format is a rationalisation of that thinking and as formats go, it has much to offer.
To be honest, the main reason I feel MFT has had such a rough go, is simply because the big three, Canon, Nikon and now Sony have so much invested in the 35mm full frame format nearly exclusively. If they really cared about a quality boost that made a difference, medium format would be more common.
If APS-C was properly supported by the big brands, it may well be that full frame would have withered on the vine by now or been relegated to “studio” camera use, but the insistence on supporting legacy formats leading to a assumption that those formats were “true”, evinced by the “crop” term used for other formats, became a watch word for “minimum required quality”.
The question is I guess, does any specific format have benefits that align better with your needs, for your work than a larger one.
A final thought on APS-C. The slightly bigger/slightly smaller format has never to my eyes proven an option, simply because it is usually a poorly supported compromise, Fuji excepted.
On direct comparison, intending to buy some Fuji a few years ago when I had some money to spend, MFT still beat it for sheer hard sharpness, which I preferred over Fuji smooth and glossy and the noise difference was not compelling, but the MFT the lens options were (I instead bought the EM1x, 300mm and 8-18, all of which have paid for themselves repeatedly).
For my own use, I find MFT benefits generally outweigh the down sides and in real terms I have never, ever been taken to task regarding the quality if my work, quite the opposite in fact. The sports team at the paper preferred my “brighter and cleaner” looking basketball files, which I joked must have been because I used a smaller and older sensor and “non issue” processing, but were more down to small, cheap, fast and sharp lenses pulling more than their weight.
Sharpness and detail are not the issue, only clean files under extreme duress, so extreme it is rarely relevant.
Full frame has it’s place in my video kit and for the occasional very tough lighting situation where needed quality possibly outstrips the realistic potential of the circumstances.