Converting Magnetism to electricity
I've been seeing a lot of videos online of people using neodymium
  magnets to make a motor spin to generate electricity.


Please link one or at least further explain what they do in the video, a lot of people do bogus videos where "free energy" is generated by magnets


  If we can generate electricity from magnets why isnt this being used
  more often?


Any electric generator uses magnets or electromagnets to convert mechanical energy to electrical energy. 


  I'm assuming there are limitations. E.g heat or the amount of energy
  the magnets can generate?


The energy is not generated by the magnet, but by the movement of the magnet and its magnetic field. You can always use a larger/stronger magnet but then you would need to put way more effort/force to move the magnet


  Can someone explain why this isnt a viable source for power?


Magnets are used on any electric generator, though they will not create energy by themselves.