If you raise $2m or $3m on uncapped, no-discount notes, you basically need to turn that into a $10m+ pre-money company upon raising your Series A. If you raise $4m or 5m+ seed, it gets even harder. And this is assuming no cap or discount, which is unlikely.
You are correct that Series A rounds are usually not smaller, in which case if you raise several million seed on uncapped notes and cannot leverage that into a much more valuable company, you'll be unable to raise a Series A.
This is all manageable by the entrepreneur, but it's important to make sure you understand what's happening and where the risks are. It seems a lot of entrepreneurs don't.