FD lenses will be optimized for 35mm cameras, which makes them ideal for adapted use on a full frame (FX) camera.
Arguably, there is another "sweet spot" when using them on an APS-C (DX) camera, because you will then only get the very best of the juicy center of the image circle.
Do be aware that a 24MP APS-C sensor is extremely demanding of lens resolution - even if we take Nyquist into account, a lens should optimally deliver good at 60lpmm to make full use of it. Also be aware that it is said that some lens designs (I do not know if any genuine Canons do) optimize for the center AND corners at the cost of the area in between (which will be your corners on APS-C) in order to get the best test results in magazines that test center and corner.
MFT is likely to be "too far away" from what the lens was built for - you will get the good stuff from the center, but you will also have a lens that is MUCH heavier and larger than it needs be, and if we are talking high resolution MFT, the resolution demands can be even higher.... Also, huge amounts of stray light from the wasted parts of the image circle, and lens designs used far far off the angle they are optimal for (eg for a 50mm lens for MFT, you would probably go for a telephoto or Sonnar-ish design, not for a Planar/Ultron type as most FX 50mm are...)
As others explained in detail, a mirrorless camera is almost always a better choice than a DSLR for adapted lenses, and generally for manual focus lenses.
You will find that original Canon FD/FL prime lenses, if they are intact, are still of great quality and can deliver results on modern digital cameras. Zooms, especially if they were meant as an inexpensive consumer option, will need experimentation. Off-brand FD compatible lenses will be a mixed bag - some will be great, but especially when you try some of the lower-tier off brands (as in, not Sigma/Tokina/Tamron/Vivitar-Series-1), you will occasionally find some genuinely bad/disappointing ones...
The one area where almost all 1980s lenses will be inferior to modern primes is absolute, global contrast; this has to do with advances in coating technology mostly...