Still, in 2003, ems might have been best practice. In the modern web, ems and pixels are equivalent when it comes to scaling. One shouldn't avoid a good tool because it's hard to learn. (Example: I use Vim for all my coding.) However, one shouldn't use a tool just because it's hard to learn. Ems, in my opinion, introduce bugs, make planning more complicated, and have no meaningful benefits.
Using ems is like avoiding functions and instead duplicating code everywhere it's needed. Ems behave differently depending on their scope, and they're a pain to keep track of.
There's a similar issue with percent-based sizing. Let's look at some css:
*{font-size: 75%} h1{font-size: 125%;}
What size is h1? I honestly have no clue without a calculator. It almost definitely contains fractional pixel value. There's no way I'm getting appropriate vertical rhythm with percents. It's the same for ems.
I might be entirely wrong. Maybe I'd fall in love with ems if I started working with them regularly. It's a personal choice, from my understanding of modern browser scaling. Pixels are a completely valid method.