There are actually good reasons why merchant accounts work the way that they do, and Stripe doesn't provide any argument for why they don't: until they do, I can't imagine trusting them for anything important at my business.
I mean, they are almost sketchy when it comes to talking about "when bad things happen"; example: when chargebacks happen (and they do happen: I get one every couple days), can I easily (as in: no human interaction) tie it to an individual payment? The only mentions on their entire website about chargebacks are in the painfully informal FAQ/pricing (that it costs $15), and in their terms of service (which indicates that the $15 doesn't include mediation, which I'm pretty certain PayPal's $20 does).
The first thing I think, of course, is "they charge more, duh": if you do any kind of volume at all, or are involved in small transactions, they are a /lot/ more expensive than PayPal: I'd easily be paying them hundreds of thousands of dollars a year for the privilege of not spending a few hours on the phone negotiating a merchant account.
(For those not looking at their pricing, their fixed fee is PayPal's worst case fee; PayPal discounts 1-2% from that for volume, and supports a fundamentally better fee schedule for payments under $12: 5.0%+$0.05 vs. 2.9%+$0.30. Unless you are handling small numbers of transactions all over $12, Stripe is going to burn you on fees.)
However, when I think through "where does the risk of credit fraud mostly occur", I'd imagine it would be reasonable sized transactions (trying to launder tons of money $1 at a time will get noticed pretty quickly), and I'd imagine the merchant fraud is mostly small volume (as if you actually have a functional business, you are able to easily able to cover the risks).
So yeah: I just don't get it... is the Stripe 7 day hold enough? (PayPal doesn't have any hold for most businesses.) I certainly wouldn't touch it over PayPal if the only reason is "PayPal seems overly cautious": Stripe simply seems to be "asking for it".
(And this is just looking at normal chargebacks: pre-orders are a whole different ballgame, and if Stripe doesn't have a policy forbidding them then they will probably be out of business, seemingly out of nowhere, in a few months.)