Flux left behind in nooks and crannies is not an issue, unless you are working with radio frequencies where it will interact strangely with the electromagnetic fields.
I beg to differ.
Flux residue is generally fairly non-conductive so any interactions with RF EM fields are generally due to its dielectric effect. Unless you are working with microwave frequencies high enough to need special PCB material or with high voltage or high power PAs, you are very unlikely to get any issues from residue that also isn't grossly visibly unacceptable.
However flux residue can be more generally harmful - apart from board corrosion leading to eventual failure, it can cause high surface leakage, resulting in problems with high impedance and low noise circuits at any frequency from DC upwards.
Rosin R and RMA flux
* remnants that have been heated sufficiently to drive out volatiles and fully activate the flux rarely cause corrosion issues, unless the resin component has been dissolved leaving metal salts behind by improper cleaning. Rosin RA flux residue can be corrosive, especially if any of it hasn't been heated to reflow temperature to activate it. RA core solder specified by the manufacturer as non-corrosive is rarely a problem as all the core gets heated to reflow temperatures simply by using the solder but you need to be cautious with additional RA flux if you are hand soldering and not fully cleaning the board afterwards.
So-called No-clean fluxes can also cause problems if not fully heated to their activation temperature + are generally much harder to clean effectively.
Water washable fluxes are best avoided for hobby, prototyping and repair work, as few workshops have the equipment required to effectively wash their residue from under components and any trapped residue is a corrosion timebomb.
* US milspec (MIL-F-14256) R flux is broadly equivalent to IPC ROL0 flux and RMA to ROM0. IPC ROL1 and ROM1 fluxes contain halides (or organo-halogen compounds that can react at soldering temperatures with metal oxides to form halides), which significantly increase the risk of corrosion if improperly washed or if unwashed in humid environments.