EDIT I realized I made a blanket claim and didn't provide any details. So I'll try to explain.
No framework, little shared code between files. Warnings and deprecations ignored, poor variable names, db queries mixed right in with html output, user id's imploded from array and stored into a single database field, etc.
Just lots and lots of rough PHP.
This isn't software to be used for analysis. Sendy is f-in awesome! I've used it for a list of over 25k and it hasn't failed me once. Like others have said, it's not as brain dead as Mailchimp, but I wasn't expecting it to be. The developer is great and it just works(tm). The price is awesome too.
I looked through the source and thought, sure, it's not computer science that was built on the latest "Framework X", but it worked fine. To me it signaled, they we're just getting shit done. God, I'm so over the days of analysis paralysis "proper framework" development (that's for another rant I guess).
Please, pay no mind to the parent post. If you're looking for an alternative to AWeber (I'm not saying it's got as many features as Aweber) or Mailchimp, then Sendy is awesome.
I've been hosting mine on a Digital Ocean 1GB server for about 7 months and it's worked flawlessly. Their 2.0 upgrade went without a hitch. Couldn't recommend this app more.
Sending email to a list is easy. Sending email to a list securely is harder.
We use Sendy at work, 40k+ sends weekly. It saves us boat loads of money since moving from Mailchimp. It's a nice thing to have.
However, we had hoped to extend it's tooling and customize, but that's right out. I've rewritten the CSV import code because it reads in the entire file, twice, before processing. I found some bugs where it can silently drop lines of your import. That's not "f-n awesome", it's bad software.
These are real things. The default Sendy setup can drop tracking opens because it doesn't access it atomically. These aren't "computer science" issues, they are evidence of a lack of competent design.
We've forked it, patched it, and use it internally, but only because it's cheaper than writing our own, for now.
Don't dismiss warnings. I'm not telling you not to buy it, I'm telling you that the guts are shitty and you should know that before you fork over cash.