Iirc there was the concern that you can stress the frame in a way it was not designed for.

 

That's why the style with the tie rod attaches to the brake boss instead of reacting on the dropout. It looks like the old Brake Therapy conversion.

The others, ugh

 

I personally would not use those at all, they look sketchy as.......
I´d stay with rim brakes before using those

 

Yes, sketchy. Just stick with V-brakes, or whatever else was on the bike...Rim brakes still actually work if you set them up right, and going to be a lot better than whatever the end result is of using one of those sketchy adapters. Or just get a new frame.

Now if you took the frame to a frame builder and had them add disk brake tabs, then I can see maybe. But not with an adapter like that, just say no.

 

I have sucesfully adapted by old MTB with a rear disc brake. I have not exerienced any issues as yet, although I am not 'sending it' down any trails at breakneck speeds or anythung remotely like downhill tracks. As you can see in the image I have used the A2Z adapter available from many cycle parts suppliers. I have fitted Shimano Mehanical Caliper Brakes.
Interestingly enough this bike originally came supplied with a front disk brake and rear Rim brakes. I have since fitted both a new Fork and Mavic Crossride MTB Wheelset inplce of the orignal wheels.

My bike is a Halfords Apollo FS26 MTB from 2004. The main frame is Aluminium but the rear triangle is made from steel

This adapter may not be suitbale for other frames as it can interfere with the rear dropout