How solid is the build quality?

basis unus wrote:

I'm currently using a 1DX and would like to have a lighter APS body for telephoto lenses. I feel hesitant about the M6 because it is said to have a polycarbonate body. I'm wondering if I could use it confidently with an EF 300mm F/2.8 lens (plus adapter). I understand that I should always support the lens with such a setup. But I think one accustomed to bigger cameras may still accidentally raise the combination by only holding the camera, since the 300mm is very handholdable. Any comments and suggestions from users of other EOSM cameras are also welcomed. Thanks!

I've handled the EOS-M5 and it felt very solid.  It certainly felt nice in the hand.  I am purchasing an EOS-M6 as soon as they become available next week.  
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I like Polycarbonate construction.  I hope you're right about the M6 having a polycarbonate body although the top and bottom appear to be anodized metal. Polycarbonate is literally bulletproof (to a 12 gauge shotgun blast at 20 feet)... which is why some types of body armor use it... especially protective glasses.  When dropped, it flexes slightly instead of breaking or denting. And it is less prone to the effects of High and Low temperatures.  There's also less thermal contraction/expansion and screws tend not to unthread with vibration. Another benefit is that the 'plastic' doesn't show paint-loss and scratches like metal does.  The original EOS-M has an alloy frame beneath the outer shell and this is what anchors the Lens Mount.  The new M5/M6 will probably have a similar construction.  The highly regarded EOS 6D Full-Frame DSLR is made of Polycarbonate components over an allow interior frame... and this is very similar.  
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You'll have no trouble using it with any large EF lens, especially with the new DPAF sensor on the M5/M6. I used a number of large and heavy lenses on the original EOSM without issue for some years.  You should NEVER use a large lens on even a high-end DSLR without supporting the glass or the lens mount may be affected.  An exception to this rule for me is when the camera is held around my neck by a strap because I will sometimes allow the lens to hang down towards the ground from the camera in this position.  The rule of supporting larger lenses applies to big bodied full-frame cameras too.  The EOS-M is so light by comparison that it balances nicely on a large lens by shifting the center of gravity.  If used on a large lens that is not mounted on a Tripod or Monopod, you will find yourself holding the lens with your left hand and operating the camera controls with your right.
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The new EF 70-300mm II lens is light and affordable.  Or did you mean the much larger white-bodied EF 300mmL lens?