General disclaimer: Not intending to turn this into a COVID debate. I'm just stating my personal viewpoint after we've been several years removed from the pandemic. Anyone can have their opinion on the matter and take the vaccine if they choose. It makes little difference to me.
In the aspect of numbers who received the vaccine, yes it's highly likely it's the most tested. At least with typical FDA approval timelines we're just hitting the minimum timeframe for development and completion of testing. Standard FDA timeline is 5-15 years before it starts being released to the general public. So in that aspect it would be one of the least tested before being released to the public. I'm using FDA as a reference since I don't know the Australian equivalent and their guidelines.
Being 4-ish years in we've probably seen all but the most obscure side effects that would develop. Looking strictly at the numbers, percentage of those who experienced life altering, chronic side affects would by most accounts be considered negligible compared to the numbers who received the vaccine. That said, I do get that looking a pure numbers is a crude way to view things when you're talking about peoples' lives, but sometimes that's how science needs to be to view things objectively. With the controversy attached to the COVID vaccine I fully understand peoples' continued apprehension about it. Even more so if they've had a negative personal experience associated with it; they themselves or someone close happened to be unlucky enough to experience one of those life altering side effects. If I were in that position there's a decent chance I'd advocate against it. I definitely do not agree with forced injections.
As far as efficacy, who freaking knows. Everything was so convoluted at that time so we may never really know. My opinion is if it's not already there it's on the way to being similar to the flu vaccine. They'll take a guess at the most likely strand(s) and hope they got it right.
Personally I've never taken the vaccine. In the beginning of everything I wasn't keen on taking an experimental vaccine and not comfortable with the risk. Now, I don't feel a need to take it since I've never contracted COVID. Same as the flu shot. I've gotten the flu all of twice in my entire life and it was pretty mild. So I don't feel the need to get one. When I make it to that age group that's more susceptible that may change.