In the transmission system (with the exception of trunk lines) direction is not relevant. Transmission is designed and built - in general in rings and power can flow in any direction in the ring.
In distribution systems there are several issues:
1) Mechanical voltage regulators have coils that can be excited by reverse power flow and mis-operate - those mis-operations can include fires in the equipment
2) the circuits are typically designed with the idea that voltage decreases from the substation to the last customer. If reverse power flow happens - that is not true, reclosers, relays and other protection equipment may need to be re-set to handle this
3) if the protection system is permissive for reverse power flow, lightning can potentially travel the whole circuit (depending about installation of lightning protection and lightning arresters) and potentially damange customer owned equipment
4) Reverse power flow can lower the fault current, if a wire is down in the right conditions of reverse power flow, the protection system may not see the downed wire and so it remains in the ground fully energized and a hazard to people, pets and property
5) Typically the conductor sizes and fuse sizes are reduced as you travel from the substation to the last customer - so putting generation in the circuit may not overload the substation transformer ,but could overwhelm the fuses and conductor sizes. Typically the best places to install large solar/generation is in the less dense population areas - which tend to be near the last customer on most circuits - overloading conductor can result in overheating and long term reductions in the conductor capacity - as well as weakening the conductor - making faults more likely
6) Reverse power flow is likely to raise the voltage locally - if the increase is enough, then the local generation will probably shut off (IEEE 1547 voltage limits) and not generate any power - resulting in unhappy owners of the generator and fluxuating voltage, since it will try to generate about every half second and then shut off again - makes the lights brighter/dimmer on each cycle.
there are other less likely issues.
Proper planining and interconnection studies can find and allow the distribution system owner to fix the issues, before they become problems.
But in general distribution was not designed for reverse power flow. All these problems can be fixed, it just costs money - in many cases far more than the value of the power being generated over the life of the generator.