I think you can overdo flavouring, but it does depend on what you are using, and what you are doing with it.
IMO natural flavours take a lot longer to 'blow' than artificial or chemical based flavours, but again there are some very very strong naturals i'd be careful with that could even make the bait repellant i.e some essential oils, and garlic particularly if using cloves.
Fairly strong natural flavouring may be the way to go if you are visiting any one venue fairly infrequently, provided it's not a flavour thats being used a lot, these days that doesn't happen too much so you should be safe enough.
Fishing a venue regularly though i'd keep your flavouring on the low side, stick to naturals, and use a couple of flavours that complement each other, so you can tweak the flavouring slightly over time.
Some years back i hammered the hell out of meat flavoured with garlic on one venue like every body else, it was the going bait and worked well for quite a long time, but eventually nobody could buy a bite on it, Switched to tikka masala paste, and the result was amazing, the curry pastes are great because you can switch from one flavour to another without dramatically changing your bait, but enough for them not to get used to it.
Add to boillie mixes as well, or soak ready mades overnight.
Hope that gives you a few ideas to work with Ramon.
Ian.