This is the problem with purchasing under-powered equipment - whether soldering irons, electric drills or motor vehicles. You can make them work ... to a degree ... but there will always be risks - usually in damaging the item and/or a poor job.
In the case of a soldering iron, large masses need a lot of heat in order for the temperature to increase to melt solder. With a decently powered iron, the iron's temperature only needs to be slightly higher, as it can continue to provide heat until the parts to be joined reach the necessary temperature.
With a lower powered iron set at the "proper" temperature, the heat it provides is nowhere near enough - and the only way to increase the heat it generates is to push up the temperature. This is a common practice - but the wider the gap between the capabilities of the iron and the needs of the joint, the harder the iron has to work which may well shorten its life because the actual heating element will need to reach a higher temperature than the tip temperature.
Aside from that, you will have to be careful about application of solder and flux. If the flux is exposed to the high temperature of the iron, it will immediately evaporate. However, the bigger risk will be if the high temperature of the iron heats components excessively. This can be through the temperature on the component being too high and/or the heat being applied for too long. The length of time is the the most common issue - just heating and heating and heating the joint until it reaches the necessary temperature. This can - and does - kill devices, not to mention PCB pads and tracks.. You will often see soldering information on datasheets - and for hand soldering it is not uncommon to see a limit of 300ºC for 10 seconds.
Yes, it can be done - and has probably been done (to varying degrees) by most everyone here at one time or another ... but a 10W iron on a ground plane without any assistance is something that is going to be an extreme challenge.