Obviously you would only use flash against the trouble fights such as whitney and chucks miltank and poliwrath respectively, however how is that disgraceful? You dont have to do it however seeing as it completely imobilises them i dont see any reason why you wouldnt.So sand attack is good now?
Flash is terrible and having to resort to it is sad. Esoecially in a game where your expected to OHKO/2HKO a good amount of the opposition. Headbutt on Meganium still doesn't help its woefully bad position and sometimes your better of using her grass STAB.
3/8?
No, she is merely decent at 3 and terrible in 5. She is a disgrace of a starter.
Also singlehandedly stopping whitney with reflect and magical leaf and potentially synthesis isnt decent its awesome. Being able to prevent primeape from using his double team strategy with magical leaf while still having the bulk to take a focus punch if need be isnt decent either. Especially when it can use flash on his poliwrath to stop it from using hypnosis. Also even though dragon resists grass meganium still really helps with the many seadras in the gym aswell as having lightscreen to halve clairs kingdas damage and flash to make hydro pump useless. It also has the high powered petaldance that can 3 hko kingdra.
In each of these cases Bayleaf/Meganium isnt decent but extroadinary against the 3 gyms that are commonly thought of as the hardest in johto.