Are there scenarios where Mega backdoor Roth don’t make sense?

31-year-old. Have 1.56M in invested accounts total. About 331K is in retirement accounts, the other 1.23m is all in a taxable brokerage. Very brokerage heavy.

I just found out about Mega Back door Roth 401(k).
I could switch 40 K a year from going into taxable to now going into the MBR.

I think it seems like a no-brainer, even if I’m going to retire earlier than 55 because of how heavy I am taxable (I have no idea when I want to retire. I like my job).

I’m assuming that I could still get my contributions back if for some reason I needed some of the money before retirement?