Replacing  the  mirror assembly in a digital SLR, makes  cameras lighter, cheaper.  But  image-wise, it 's no drawback.
By Anand Parthasarathy
May 20 2019: Mirror, mirror on the wall, which is the best camera tech of them all?  That's a tough one to answer, especially if one of the choices has ditched the mirror itself.  The Single Lens Reflex (SLR) camera or its film-less  successor the Digital SLR or DSLR, remains the most trusted camera format of the professional photographer.   Serious  artistes  insist on  seeing exactly what they were shooting before they pressed the trigger. To make that happen, the DSLR uses a mirror to shine light from the full frame of the optical viewfinder  to the eye piece. Once you are satisfied, the mirror was retracted  before the shot was captured.  All this called  for  bulky and additional mechanical parts and springs.  
If you  insist on "what you see, is what you shoot", and want to do that with a variety of  lenses from close-up to telephoto -- there was no option to the DSLR -- till now.  Major DSLR makers  have began offering  an option,  much lighter,  somewhat cheaper  and  almost there when it comes to seeing what you shoot: they removed the mirror assembly and used the same  imaging sensor to  preview  the image and capture it.  If this sensor is as big as the image captured, they called it a full frame mirrorless camera.
Small downside:  in low or bad light, the previewed image was somewhat dull in the early mirrorless cameras -- but technology is fast narrowing the gap.
And  one upside:  these days all digital cameras shoot video --  many to ultra HD or 4K quality.  And with video, a mirrorless camera   performs even better than a DSLR .... and also provides better burst photos  (up to 5 per second).
The biggest advantage of a mirrorless camera over the point-and shoot  digicams and indeed all phone cameras, is that you can switch lenses just like a DSLR pro.
Last month  both Panasonic and Canon  launched  mirrorless cameras in India.  Panasonic offered the Lumix S1 and SR with 24 MP and 47.3 MP  sensors and a 24mm -105 mm lens.   
Canon has brought its second mirrorless full frame camera, EOS RP,  to India  and provided me  an early  opportunity to try it out. Please see  Product Pulse  for a  review