Depending on how you want to look at it there are 2 or 3 types of networks on the grid depending on perspective.
On way is by looking at the transmission grid and distribution grid. On the transmission grid where you see high voltage lines and generators and power flows both directions withing this network. You are correct in this respect. The problem arises on the distribution network side where instead of high voltage lines and generators you have customers and low voltage stuff. Here things are basically designed to flow one way, which is to the consumers.
The other way of looking at it is with 3 networks being high, medium, and low voltage. With this view things get a little more interesting as there can be bidirectional power flow within each level but it becomes problematic when it is between levels.
With either view of the power grid it really isn't too big of a problem* if you or a few nearby people are providing excess power that your neighbors are using as you are all on the same substation that is fed with medium or high voltage lines and provides you with your nice low voltage power. The problem is that if too many of you are feeding power back into the grid it may outstrip demand and now instead of that substation taking power from the medium voltage network it now is trying to push power up into the medium voltage network. This is not what the current grid was designed for and the equipment at the substation while it can do it doesn't do it well. The same thing can happen between the medium voltage and high voltage networks although it is rarer but has happened. This also ignores the grid management aspect of things which is all in software and is basically a traveling salesman problem solved as best as it can be continuously.
These are not unsolvable problems but instead are engineering ones that people are working on. Companies are already designing better switch gear, beakers, transformers, etc to handle bidirectional traffic. The modeling, management , and market applications are being developed to handle many more points as well as having them be bidirectional. Granted these now require substantially more computing power but technology has progressed where getting that computational heft isn't an issue.
*The one issue you have with large scale intermittent distributed power (rooftop PV) is what I like to call the rouge cumulus cloud. It is a nice sunny day and he decides to blow in over your neighborhood, and then out. All of a sudden your local substation goes from pumping power out to sucking it down, then back to pumping it out. It doesn't even have to be this severe, just going from low draw, to high draw, back to low draw presents similar although not as severe problems.This is murder on equipment and a real bitch to deal with from a grid management perspective. To prevent this some local grid level storage at the substation would help to level the load making it much easier to deal with. So again not an impossible problem but an engineering one.
