It is described as a hi-tech blanket that will give you “the best sleep ever” and reduce your stress levels, though at about £195 ($249) a pop it is certainly not cheap. The Zen Blanket was promoted on the crowdfunding websites Kickstarter and Indiegogo, prompting thousands of people to sink money into the venture – but some are now crying foul, demanding their money back and discussing class action lawsuits.

One of those who is definitely not feeling zen is Don Dennis from Scotland. He pledged £118 to the project, in return for which he would be sent one of the blankets (£195 is the usual price, but early-bird backers received a discount).

With the project dogged by delays, prompting growing anger among backers, he was left wondering when, or if, he will ever get his blanket.

The company behind the product says: “We are doing our best to deliver the rewards as soon as possible,” adding: “Rest assured that no backer has lost any money.”

Dennis’s experience highlights the risks attached to putting your cash into crowdfunded projects of this type, although millions of people globally have backed a vast range of ventures, and there have been numerous success stories.

Dennis, who lives in Argyll, says he put in an order for a Zen Blanket earlier this year after seeing it promoted on the US-based Indiegogo site.

The brainchild of a Swedish startup company called Z Trading Handelsbolag, it is described as “a weighted blanket backed by science that improves your sleep while reducing your stress”. It contains heavy plastic poly pellets, so that it presses down on your body, thereby apparently simulating the feeling of being hugged or held. The Zen Blanket was offered in four weights: 3kg (7lb), 7kg (15lb), 9kg (20lb) and 11kg (24lb).

Lots of people signed up: the Zen Blanket page on Indiegogo says about $805,000 (£635,000) was pledged, while Kickstarter says 4,150,000 Swedish kronor (£360,000) was raised.

Dennis, who runs a dairy business with his wife, handed over his money and the Indiegogo page told him the estimated delivery date for his blanket was October 2018, but that month came and went with no sign of his item and Dennis says he has heard nothing.

On Kickstarter, the company has provided several updates: in August it said that because of the huge demand there would be a delay, and the plan was to have the blankets delivered “before Christmas 2018”. But on 31 October the company said the new estimated time of arrival “is tentatively set to Q2 2019” (between April and June 2019).

Scores of unhappy backers have posted comments on Kickstarter and Indiegogo, with “When do I get my money back?” and “I’d like a refund” among the more publishable ones.

Dennis is worried he will end up with no blanket and no money, and says that what makes him particularly angry is that the two crowdfunding sites “don’t seem to be particularly fussed”.

He did receive an email from Indiegogo dated 29 November that read: “We understand there are concerns about the Zen Blanket campaign and we are currently investigating this issue.”

Indiegogo and Kickstarter have both benefited financially from the success of the Zen Blanket fundraising campaign: the two sites each charge a fee of 5% of the total funds raised.

When Guardian Money contacted the company behind the blanket, a representative, Philip Johansson, said that when the campaign was originally launched the company had expected to receive 100 orders, but the demand was much greater than anticipated, and it had ended up with 5,200.

He added: “We underestimated how much time projects like these take. Our estimate is to be able to fulfil the rewards in Q2 2019.”

Johansson said some other rewards-based crowdfunding campaigns had experienced “much bigger delays” than it had.

On Kickstarter, the company added: “We are unable to provide refunds and we do not have a refund policy.” Indiegogo told us it was looking into the Zen Blanket campaign, adding that it no longer had access to the money paid by backers. A Kickstarter representative said: “I have looked at the updates and comments on this project and have noted that its creator has been quite active in explaining the situation and in responding to questions from backers.”

Crowdfunding explained

So how does this sort of crowdfunding work?

Kickstarter and Indiegogo are the two biggest players, and are basically platforms that allow creative projects – from the latest technical gizmos to works of art – to get funding. There’s a huge choice on offer: for example, on Kickstarter you can currently back a new eco-friendly and vegan dental floss called Zauberfaden where pledging €9 (£8) will get you two packets of the floss. Or stump up from £17 on Indiegogo and you can get get an early-bird version of “D,” described as “the first reusable tampon applicator”.

Both sites argue that contributing to a fundraising campaign is not the same as buying a product, but in effect that is often what people are doing: they are handing over money, in return for which they get a “reward” (in the case of Kickstarter) or “perk” (Indiegogo), which is often an early version of the product.

Is it big business? It seems to be. Kickstarter says that since its launch in 2009, 16 million people have backed a project and $4bn has been pledged, while Indiegogo says its group of backers is more than 9 million strong.

Do projects sometimes fail?

Yes. Kickstarter invited a University of Pennsylvania professor to conduct a survey, and he found that 9% of projects failed to deliver rewards as promised.But he added: “Ultimately, there does not seem to be a systematic problem associated with failure (or fraud) on Kickstarter, and the vast majority of projects do seem to deliver.”

Indiegogo says the reality is that any business might fail, and if you want to contribute to an innovative project, “it’s a risk you have to take”.

So there are no guarantees offered? That’s right. Kickstarter says it “cannot guarantee creators’ work,” adding: “Responsibility for finishing a project lies entirely with the project creator.” Indiegogo says it “does not guarantee that contributions will be used as promised, that campaign owners will deliver perks, or that the campaign will achieve its goals”.

Are refunds ever available?

Kickstarter says it “does not issue refunds. Transactions are between backers and creators directly.” But it adds that if the problems are severe enough that the creator can’t fulfil their project, “creators need to find a resolution. Steps should include offering refunds, detailing exactly how funds were used, and other actions to satisfy backers”.

Indiegogo’s policy seems a bit more user-friendly. It does offer a refund policy, though it only applies in limited circumstances: for example, the crowdfunding campaign in question must still be live, and the funds can’t have been sent over to the project creator.

However, Indiegogo’s small print states that the legal obligations of companies that are raising funds include a requirement to “issue refunds to contributors if you cannot deliver perks”.

Indiegogo adds that if a project creator doesn’t fulfil its obligations, “we will take appropriate action … We will do everything in our power to recover funds for our backers, including contacting third-party collections agencies”.