I've been generally lucky on my kickstarters, but I also tend toward backing only the digital version of TTRPG things, with some exceptions.
While there are a few that have had issues, or slow, there's only a few that have failed in part or in whole...
SpaceJammer 5E - Hardbound Support, by ArcKnight. The digital portion has been delivered, but they went radio silent afterward for months. Partly because the subcontractor responsible for the digital portion left when their part of the job was done, and partly because the ArcKnight owners lack of desire to give a concrete deadline - which kept slipping, not because the materials weren't done, but because the box manufacturing kept hitting a snag. Their last communication swore they'd be better about communicating any changes. That was in August, a year after their original delivery date. Still no word. ArcKnight standees are still apparently for sale through their site, but not responsive to emails.
Vagabond's Guide to Dalriada, as another said, this. While I followed the ups and downs of their company through updates which appeared to be caused by interpersonal issues, and individuals exiting said company. It's been radio silence for a while now, and we are 2 years past delivery of anything. Their prior short adventures were simple, straightforward and good, and I wasn't worried, but should have been concerned when they tried to have 3 large volume kickstarters at once.
Bergin's Book of Beasts - DnD 5e, from the same folks as the above, was late due to the same issues, but at least from a digital perspective delivered eventually. It was technically what it claimed to be, a volume of stats for natural animals - but was taken to it's most boring interpretations across the board, and frankly was not what I'd call a useful tome for 5E GMs or druid players overall.
Rewards & Rarities, from 2CGaming, I got a beta PDF, aka The Community Copy, essentially asking the backers to read through it and send any feedback about it for the final polished version. That was in Oct 2023, with the original delivery of everything set for July of 2022. That was the last update, and 2CGaming would not reply to any KS messaging nor to direct emails to their site. While there was some interesting stuff in said PDF, it seemed scattered, like it was unfinished, or that someone was hacking and slashing the original writers content together.
The rest of the delayed TTRPG KS or Backerkits I funded delivered. The score or so that were late were usually less that a year, were still good products, and the creators did their best to remain communicative. So, for a supebacker with 126 under my belt, I feel like I've been remarkably lucky in that regard.
docs.google.com
