How bad is it to start running FB ads only with PayPal Express Checkout? We want to sell only in the USA.

Paypal by itself is fine. I know people who do well just with PP. Most buyers in the USA have a PP account. Be aware of Paypal holds please. ALWAYS upload tracking numbers for every order, submit your business information to Paypal to verify your identity/ company name and DON’T extract big sums of money in bulk from PayPal, do it in small amounts to your bank account if needed.

Extracting big sums triggers the PP safety system and they might put your account on hold. Stripe by itself is also tricky to use. ALWAYS let your customers know how much time it will take to get the order with an order confirmation email. Confirm your business information if Stripe requests it and DON”T scale too aggressively using Stripe only. You might use https://paycertify.com/ as an alternative too, basically doing half of the volume on 1 payment processor and a half on another limiting your risk.