1) If an investor invests $500k at a $4.5MM cap, he signs up to get 10% of the company assuming the Series A priced round will be at a higher valuation. But, if Series A is at a lower valuation, say $2.5MM, then the investor gets 20% of the company. However, if there was a discount associated with the convertible note then the note converts at 80% of $2.5MM. So a down round is really bad for the founder. But I can't imagine a down round being much better for a priced round.
2) If the Series A is $3MM at $12MM pre with 1x liquidation preference, the Series A investor gets 20% of the company. The seed investor who invested $500k will have his shares converted at the $5MM valuation. However, he will end up with a 3x liquidation preference or $1.5MM in liquidation preferences. I think this is okay as long as you raise a few hundred thousand dollar convertible note seed round. If the convertible note seed round ends up to be in the $1MM range I think this can become an issue for the Series A investor which is Suster's main point.
Convertible notes have benefits like high resolution financing, less control & no board seats. I still think convertible notes are the way to go if you are raising a few hundred thousand dollars in seed. But if you raising a seed in the $1MM range, it seems like priced rounds are preferred.
re high resolution financing: if you are offering different investors at different prices, you are likely to make your investors less than happy with you.
re less control and no board seats: in my experience, having a board correlates strongly with success. so i think this is a negative. to put money where my mouth is - for my last startup, I took a board even though I did not have to.
re raising a few hundred thousand or less: in my experience, strongly correlated with startup death.
re less control and no board seats: agreed. though i think it depends on the board member. in your case, i'm sure he was top notch :)
re raising few hundred thousand or less: hmm, don't have enough data. but uber's initial round was $200k. https://angel.co/uber