The downside is this guarantee has some trade-offs. Mainly transaction speed is super slow (10m to confirm a transaction went through) and transaction fees are very high (especially now with the "bubble", it's something above 10$/transaction). Furthermore, all transactions are public record, which isn't desirable if you're a fan of privacy. The value is very volotile right now, making it hard to really budget out your net worth.
IMO, if you can just trust a third party (eg, credit card processing), then you get much better guarantees. (Fast payment, relative privacy, low transaction fees), with the further benefit of chargebacks. If someone steals money from your Bitcoin wallet, you're screwed, with a credit card you phone them up and they undo it.
Actually, even with Bitcoin you need to trust a third party: the exchange. Many people hold their money in the exchange, and there have been multiple cases where they've lost a bunch of user's money. IMO a real bank or credit card processor is more trust worthy than a crypto exchange.
That's all to say that currently crypto makes no sense for real transactions. That's not to say the issues are unfixable, for example Monero provides privacy guarantees.
As well, other uses may be valid, the main case is "a hold of value", like gold. IMO, crypto has no intrinsic value (not even decorative like gold) and is very volotile, so it's up to you to decide whether it's going to last.
Lastly, there are novelty uses like ethereum's trusted computation platform. That's a complicated topic in itself but pretty much each of these uses has a counterpart with much better guarantees if you can use a trusted third party.