There are good and bad points for overselling.
PRO: You can earn more money--for both host and client side (oversold hostings are also usually cheaper)
CON: When most of them suddenly have a surge in usage
When you meet clients who optimize the resources given to them
Resellers cannot upgrade servers to prevent it from crashing.
Conclusion: There are more risks in overselling (and even more if they oversell too much).
I don't know how long you have been doing any hosting at all, but you have missed some studying for sure.
Dedicated servers whether used for shared hosting/reseller hosting all have definite resources that can't be easily upgrade for the most part.
I have seen a many dedicated servers crash under load. You must have not ever heard of DOS/DDoS attacks. Which really is no different that getting a load of traffic the only difference is the intent.
I have seen many sites that were run on dedicated servers go to a crawl after being announced on the TV for some special purpose.
VPS/Cloud Servers are the only structure you can easily upgrade to match the demand.
The one option to upgrade that can keep a Cloud Server/VPS down for a while is adding more disk space.
Now I could be wrong about all that, but I have never heard of hot swapping RAM just hot swappable drives.
Point is that being a reseller doesn't mean you can't upgrade nor expand.