I'm not sure if I would prefer EF-M dedicated Samyang 10/2.8 over the normal one (with EF mount) + adapter.
Since I already have the adapter I think I'd go for the EF mount after checking the specs on the 16f2, http://www.syopt.co.kr/common/pdf/f=16mm.pdf. The EF mount is 585g and a length of 89.4mm while the EF-M mount is 615g and a length of 115.4mm. You don't get a smaller form factor for the M, you just get a built in adapter. The EF-M mount is about 75g lighter than the combo if that makes a difference. I assume the 10F2.8 would be a similar story.
The 300mm is a more compact design at only 320g and 73.7mm on the M, http://www.syopt.co.kr/common/pdf/f=300mm.pdf. It doesn't have an EF equivalent. Maybe there will be other lenses like this in the future targeted for the smaller form factor bodies.
And that is going to be the basic problem with 3rd party options. The lenses designed for M specifically benefit from being designed for the flange and are nicely compact. Lenses that are designed for APS-C with an M mount are not really specifically designed for the body and won't have the great small form factor. Still, better to have some options than no options...and Canon hasn't exactly been pumping out the lenses.
Yes. Perhaps that's because Canon is marketing it as a pure consumer product (EF-M system), so 1-2(3) lenses is enough, and if you are an enthusiast, then you must add the EF-to-EF-M adapter, or skip the whole thing and get a Rebel (which lacks a lot of dedicated, affordable primes as well). I think that Canon non-professional product segmentation/politics is a big mess, god help them ...
Their only APSC DSLR UWA lens is the EF-S10-22USM and they are squeezing this lemon for 10 years. It is decent quality, but now they are expecting Americans to use it on EOS-M or something? Who's making these crazy decisions?
They should learn from Sony - "If you can't make something, buy someone who can".