Well I have owned many cameras and currently got a Sony A7s (Full Frame), Panasonic G7 (M4/3), Panasonic G6 (M4/3) and an old Canon EOS 550D (APSC). I am not going to talk about in-depth differences of noise and dimension as there are plenty of info you can find of those. I am just going to talk about my opinion and real world uses of them. My opinion is that they are all great cameras for photography and the smaller sensors does not really matter that much in real world uses. As I have shot plenty of great photos that were shot on the smaller sensors and nobody would see that it was shot on a smaller sensor. This shot below was taken with a Panasonic G6 using simple LED panel and this image could been easily used for an advert.
Studio Photography with lighting and flash
With proper lighting and proper flash setups you can take great photos, even with your phone. And to be honest for any photographer who want to exceed in their photography game, need to work with external flashes, as you cannot rely on just taking photos with only natural lighting to produce professional pictures. This image below looks like it was shot in a studio, yet this was shot during a busy convention and had a lighting setup in a small corner of the event to take these kinds of high quality portraits, this was shot on the old Canon EOS 550D with Canon 24-105mm f4 using two Yongnuo YN560III and a Phottix softbox in a beauty light setup.
Eventhough the location got bright with TL lights on the ceilings, but its not that flattering of a look and using flash I could easily just cancel out those light and replace with strong speedlite flashes and paint the person with lights how I want it. Especially when you go clubbing the amount of moving bright spotlights and moving people make it really impossible to capture a photo with just natural lights unless its some lucky shot. And mainly allot of studio portrait and fashion photos you don’t need to get a blurred background as the background often a flat surface where you don’t see much bokeh and often need the whole person in focus. And I use the 550D for this a using flash would need to use the physical shutter and rather use up the shutter on inexpensive camera than on an expensive one, plus I needed to cancel out the natural light and able see what I am shooting and on Mirrorless would show me a black image through te viewfinder and on a DSLR could still see what I am shooting and frame for it.
Landscape photography
Ok, the A7s have killer low light but really its only when you film with it for photography the RAW files are quite noisy only the JPG are much cleaner. Taking landscapes on the other hand natural light is needed, but small sensors nowadays can handle it well and you could use a tripod to do long exposures with them. But nowadays with mirrorless cameras you turn off the physical shutter and shoot at 1/6 handheld easily, which was done in the image below, was just shot out of the hand and no tripod used. This shot was taken with Panasonic G6 with a Canon 10-22mm f3.5-4.5 lens with a speedbooster and still take good low light images. And really who need thin DOF for any landscape photography so the tiny sensor help getting more in focus even with an wide aperture.
Also for interior or landscapes APSC has the least expensive ultra wide lenses you can get. As the 10-22mm f3.5-4.5 is relative affordable but the new 10-18mm f4-5.6 is really inexpensive. As if I want to get the same FOV with a native M4/3 lens, I would be looking at spending $1000 on a 7-14mm F4 Panasonic lens. And on Full frame be looking at either the Canon 16-35mmm or 17-40mm lens which both are quite expensive. Which is why I opted for a 2nd hand Canon 10-22mm f3.5-4.5 with a Chinese speedbooster.
Product Photography
So you might think taking product you need nice blurred background and such, but allot of products are so small and need to take photos close up that mainly a Full Frame just does not make sense to take the photo with, as the DOF be so thin to see most small product perfect in focus. For product photography I would gladly grab my Panasonic as they have more in focus, as this picture below is taken with Panasonic G7 with cheap Fujian 50mm f1.4 lens on f5.6, if I had used the A7s I would had needed to up the aperture to f11 or even higher to get most of the camera in focus, but would also needed very strong light compared to a simple LED panel I used to get the lighting of how I wanted it.
And taking Macro photos of details of the camera you can get more of a close up with a M4/3 camera using the same lens then a Full Frame as the 2x crop makes the image zoom in twice as much. As the image below was taken at 300mm macro with the Panasonic G7 and if it was shot with an full frame it would have been a much wider shot then what I wanted.
When would I use a Full Frame?
For portraits where I need to mix natural with speedlites, I would go for the Sony A7s. The reason for that is flash has a certain sync speed of 1/200 which is not that fast in the full sun and still easily overblow the image. With a Full Frame you a much thinner DOF over the smaller sensors, as if you look at the image below the background is blurred and this was shot with Sony A7s with 28-70mm f3.5-5.6 kitlens on 70mm f5.6. 
To get a blurred background on a smaller sensor I would probably needed to grab my 50mm f1.8 lens on the APSC to get a similar FOV but also needed to shoot at ruffly f2-2.8 got the same blurred background, but would had needed a higher shutter speed or needed to use an ND filter. Even though there are also HSS flashes that can up the shutter speed, but they are quite expensive compared to non HSS flashes by quite allot, as I use relative inexpensive Yongnuo YN560iii which are about $69 each with a YN560TX to control them that cost $36, but if I want a HSS for my Sony A7s I would be looking at Phottix ODIN trigger set for $299, which is only a trigger and still need to buy flashes. But there is a major difference in the lens choices also, as for portraits using a 200mm lens get you really nice bokeh, shown in the image below.
Which is on an f5 for a Full Frame, which there are quite allot of affordable tele zoom lenses that could achieve this. As this was shot with a relative cheap Tamron 70-300mm f4-5.6 that cost $150. If I needed to replicate this shot on an APSC I would had need to use a 135mm f2 lens which is quite pricey. As with pretty much an expensive Full Frame camera you can achieve great thin DOF photos with quite affordable lenses, yet with smaller sensor you are paying more for allot of expensive lenses to achieve thin DOF shots or need to use even more tele lenses but stand really far away and often indoor photoshoots of portraits often just does have enough space to do so.
Conclusion
When you want to achieve thin DOF a Full Frame give you benefits in allot of way, but in most cases and occasions you don’t really need to have such a thin DOF and a M4/3 or APSC would do the job just as great, but you can achieve the same result of DOF with also the smaller sensors but compromise in more expensive lenses or having to stand further away. The sensor size does not really defines that a shot is more professional or anything as most people don’t even notice on which sensor the image was shot. But smaller sensor are great when you need to have more in focus for small products, but also when you are taking a picture of sporting events you can get a much more tele shot with the same lens then on a Full Frame. The different sensors just have their own benefits and its just your own choice what kind of photography you do the most to choose the best that fits your needs. And just don’t be tempted by people saying one is more pro than the other, as you need to be the one who should decide which really fits your needs.
