It should be OK imo, I've rejoined wires with as little as a windproof lighter and some solder on a field before, the issues it could cause however are as follows, a bad solder joint can increase the resistance in the wires it's on which can cause the solder joint to heat up under full auto and potentially break the connection, the increased resistance can also make your gun run slower sooner as the battery's voltage decreases, this shouldn't be a massive issue though as it's not likely to destroy the gun or make it explode, if it feels like it's dying quicker than normal consider refreshing the joint, the best way to rejoin them would be to tin both ends of wire with solder (use high temperature solder with a flux core if you can), then using the iron get them both nice and hot, (I often use clothes pegs to hold the 2 wires stable whilst I do this) and once hot. Dab a little more solder on to flow the joint nicely, finish up with a good blow, some heatshrink or sparkies tape.