Про LTO нашел кое-что интересное (конец 2019)
> It makes your application substantially harder to debug. This is getting better in newer releases but after an LTO pass, a lot of debug information cannot be captured due to the nature of the optimization. If a chain of seven functions across several c files is only called once, LTO may chose to inline them all together. If you crash somewhere in the seventh function, the backtrace may just show one function instead of all seven.