I've used this program for years. It's quite feature-filled and useful.
I use this sparingly, not every day. So sometimes I forget which settings work and which do not. This has led to two high-stress problems for me recently.
First, I always forget how to get attachments to work, and I struggle to format the .csv file correctly. The best luck I've had is to make sure that a double-quote is around every text field in the csv file. But I've had trouble getting even this to work before.
The second problem sent me in a panic earlier today. To my memory, I thought that the default send setting on mail merge was to save everything as a draft that I can review before sending. I realize there's an option to select this, but I thought it was the default, and I ended up sending a bunch of letters I didn't review. This turned out to have fairly bad consequences in this case, actually, and in any event was an unpleasant surprise.
These are really my problems, and shouldn't reflect poorly on the program. But it could be improved by making it more idiot proof: making "save as draft" the default setting and having a simpler way to associate file paths with other data for attachments.
This review is for a previous version of the add-on (4.11.0).