Seizing the initiative

I was thinking, would it make sense to have a drawback to failing to seize? Like being forced to put x units in reserve if you fail? Or is that an advantage even?

I don't think you need a negative for trying. It's a tactical decision depending on the game type, I mean, you could potentially catch your opponent off guard and get a decent alpha strike, but you might be giving up the last say on objective grabs/contesting at the end of the game turn.

Tl;Dr sometimes going second is better than first

Yeah maybe. I think I shoulda gone second in the last game we played yesterday.

Yeah being reactive with deep strike reserves (oblits vs oblits, or fliers vs fliers) isn't ever horrible either

I think seize is really the only thing stopping someone from deploying for an alpha strike every time. I don't seize should be thought of as a beneficial factor, it's more of a mitigation of the benefit that having first turn gives.