[JRTJH]

The only problem (actually not a problem but an accident potential) is backing onto the "approximately 5' ramps). As long as your 2x12's are on firm, level ground, you won't have a problem, but if you back onto the "long" ramps in soft soil or on sand, you can back the first tire onto the 2x12 and cause it to "bury in the sand" which kicks the other end up, catching the trailer frame, wheel well or fender skirt (depending on how unlucky you might be). That can cause the 2x12 to become a "puncturing ram" depending on how far you keep backing, the angle of the 2x12 and your "luck".... Pulling off the ramps can cause even more significant damage as the last tire off the ramp will "lock" the 2x12 to the ground as you continue pulling forward, pushing the wooden ramp deeper into the wheel well increasing the damage.

There's no problem using 5' ramps as long as you pay attention and are on firm ground, but they can cause significant damage if they "kick up" when backing onto them. That's why all my 2x8 ramps are for single wheels and are cut to 1 foot lengths. YMMV