The best antenna is a 1/4 wavelength whip in the middle of your roof. There is no argument on that. Other good spots are flat spots, like trunks or even hoods (isolating from ignition noise is a problem there, though).
But, that is not always practical. I personally have about 8 inches between my roof and garage door header. I put my antennas on my ARB upright and get fine performance. My pattern is obviously heavily biased towards the back, but it works pretty well in that direction. Think about this, lots of motorcyclists use antennas on their bikes, which have very little chance of large flat metal places. Sometimes they use cargo boxes with sheets of metal, but still that is not great. So figure out the antennas they use and they are typically pretty decent for bumper mounts.
My mount is a 4" piece of aluminum that is held onto the round tube with clamps. That aluminum is grounded to my bumper with a piece of braid. The bumper itself is then grounded to the frame with 2 pieces of grounding braid (never rely on the bolts, the anti-seize and powdercoat pretty much eliminate any chance of conductivity). I also ran a heavy braid from the battery to the frame, along with ground straps from the hood to firewall and front fenders. I run a Comet Challenger C-767 dual band, which is a 1/2 wavelength center loaded on 2m. It is not an elevated feedpoint 1/2 wavelength (which is completely ground plane non-dependent), but it seems to be less reliant on the counterpoise none-the-less.
My point is this, a bumper mount can be acceptable, but you have to take the steps to ground everything in the front and still you are going to give up range (especially to the front). By grounding I mean RF, too, not just electrically (they are different). The compromise for me was OK. I had a roof mount CB antenna for a while, but it got to be a pain taking the whip off to park and it got knocked off a lot while on the trail. A hard mount would have helped that, but still it gets whacked a lot and that's not good for the whip either. So I decided to use the bumper and did all I could to make it work. I personally like having the 2 whips on the corners of the front of the bumper. I bump into less stuff when I park my truck now, they act like markers for me. FWIW, on 2m at 50W, I typically get around 45 miles range in mixed terrain with decent strength (enough to open a repeater and still be understandable, although not necessarily super clear). To the front I get around 30 miles with open terrain or good position, maybe 25 or so in mixed terrain. For me, the compromise is worth it, but I'm not a super ham hobbyist, just another way to communicate.
