What sort of issues have the other author's had?
The issues about which people most commonly complain are poor delivery rates to subscribers' in-boxes, rather than to their "spam"/"junk"/"bulk" folders (people who don't test, reliably and frequently, themselves, by subscribing to their own lists from several different email addresses and using different email client software tend to be unaware of this: it's something of a "marketer's trick") and extremely idiosyncratic and inconsistent interpretations of the terms of service (which often come to people's notice only when their accounts are suddenly terminated without notice and without their understanding what they've allegedly done wrong).
That last point, on the web, is a problem frequently associated with the use of "free services". It's
much better, in general, to be either a paying customer (in practice, this tends to give you some "rights") or a self-provider of services (in this case, that would mean installing and using a self-hosted autoresponder).
I believe (but can't offer any direct evidence for this belief) that Mailchimp is especially trigger-happy when it comes to closing down the accounts of their free users who ever get any "spam complaints". These are always a potential problem, and especially for service-users who send infrequent mailings and/or "broadcasts" only rather than scheduled email series (and Mailchimp's
free service is limited to "broadcasts" only!). Some people are careless; others imagine that clicking on "spam" is the same thing as clicking on "unsubscribe". Unjust it may be, but the reality is that list-owners commonly suffer from mistaken subscriber identification of their emails as "spam". The ways to avoid this are (a) to send email regularly and reliably, so that subscribers never forget who you are and that they opted in, and (b) to use email series rather than or in addition to broadcasts (another good reason for avoiding Mailchimp).
There's an additional problem, too: no serious, professional autoresponder company will allow new customers to import a list built at Mailchimp without all their subscribers re-opting in. In reality, an attempt to migrate a list from Mailchimp to another service provider is likely to lead to a loss of anything between about 50% and 75% of the subscribers. The idea of "starting out with Mailchimp's free service and switching to something rather more professional when one can better justify the cost" is, therefore,
deeply flawed.
In summary: this is one of the very few issues in "internet marketing" which it really does pay to "get right" the
first time you set it up: unfortunately there's a considerable opportunity-cost to "learning from experience", in this regard.