Lots of drawback for this. I think too many to list, and some of them are subjective.
First, if you want to improve the performance of the front end, you don't just change one variable (ie fork length). All bikes are specifically designed on purpose and if something doesn't work well, there are more fundemental problems than that.
My first suggestion if you want to improve the bike, would be to see a qualified person that has lots of experience on that specific type of riding. Not any mechanic or random person.
Different travel bikes have their specific amount of travel for a reason, the harder hitting, the bigger travel (to a limit), the less extreme, the less travel. If you change the amount of travel of a bike that was suited to 100mm, you will take away a lot of it's specific properties, ie it's bump vs pedalling efficiency.
You add 40mm travel and set the fork how it's designed to be riden (ie correct sag etc). What you have done is drastically changed the geometry of the bike. Whether this is good or not, is subjective. The fact is the bike will desend with more stability but will pedal badly and corner much worst. Not to mention the extra stress on the frame which may break it.
You add 40mm travel and set the fork so the geometry of the bike is the same (ie very under pressure). What you have done is put a poggo stick on your front end. Yes, you won't feel small bumps, but you won't be able to stop bouncing.
The take away message should be, either improve your current bike by properly fine tuning your fork or upgrade to a better, more suitable, fork which does what it's supposed to do.
On the other hand, you might actually need more travel, as your type or stype of riding dictates that, then you should get a different bike more suited for your riding. You don't take an XC bike to ride enduro or visa versa.
I learnt a lot about this watching Vorsprung Suspension on YouTube. Some of it gets pretty technical, but it's fantastic to learn.