Can anybody explain me the DISadvantage of e-Newsletter...?

Hi,

read about the e-Newsletter today.
This plugin looks nice!

Can anybody explain me the possible DISadvantages of it?
Some "big" companies warn about sending myself, for possible problems like deliverability problems, spam filters, and so on...

Is that true? How effective can I send Mailings with e-Newsletter and are there potential problems? And how many mailadresses can it handle before getting into trouble?

Thanx for your answer!