No they aren't. There are a lot of disc only rims that are rounded quite sharply where the rim brake track should be. You can't use those rims on a rim brake bike. So look at the pictures to ensure that the rim has a nice flat section right at the tire bead. Ideally you'd search for the rim's maker and model and it'll tell you in the write up or specs or by the look of the cross section diagram if the rim will work with rim brakes.
It used to be that mountain rims were mostly rim brake compatible. But looking at the major makers they are offering only half or less of their total MTB lineup that are rim brake compatible now.