I like how easy it was to set up our email templates, which we have been using in house since 2011 with no issues or complaints! The customer support team are amazing as well; they were able get us sorted out very quickly (within hours!) when there had previously only ever being one person available at any given time - this makes such difference from other companies where you need months before someone will even reply.
And finally:- if your sending large numbers/quantities emails then yes some servers do struggle but not ours thankfully :). Nothing really though just general ease setting things all around so far haven't experienced anything majorly wrong yet :) Great service overall!! We use bit faster primarily because its integrated into Zapier allowing quick automation between platforms without having 2 separate apps / systems etc..