Amphibious operations should be harder than normal operations. They require multimodal logistics (which needs to be protected) and force the landers to fight outgunned and outnumbered during the initial phases. But the thing is, how hard they are is probably an exaggeration. There are many successful amphibious assaults in history such as:
- Turkish landings in Cyprus
- French-British-Israeli landings in Egypt
- Iranian landings in Iran-Iraq War
- American landings in Korean War
- Entire WW2 Japanese SE Asia campaign
- Entire WW2 American Pacific campaign
- British landings in North Africa and Madagascar in WW2
- Allied landings in Southern France and Italy during WW2
- Soviet landings in Crimea in late-WW2
- And of course, the D-Day. The Normandy Landings involved more troops than the totality of the current ROC Army.
Also, non of those assaults involved the attacker landing multiple times the defender's military in a single day. Which, bizarrely, people think China needs to achieve against Taiwan. I wonder if they think you need to outnumber the other side in the first day or your soldiers instantly die?
Come to think of it, there actually aren't that many examples of failed large scale amphibious assaults in modern times (since 1900). Battle of Gallipoli and Battle of Kinmen are the only ones that I can think of.