I haven't used Saily, but I've heard good things about it. I think it's like other eSIMs, and if the price is right and it covers the countries you want, then it's certainly a good option.
I've been using Bcengi, most recently on my last trip in Norway and like it a lot. It's a pay-as-you-go plan, which means you can keep it on your phone and use it whenever you travel. When you don't use it, you just turn it off, and so you only pay for the amount of data that you use at any given time.
The nice thing about it is that it covers all countries, rather than you having to purchase an eSIM for a particular area or country. The price for data varies depending on the country, but they publish the prices on their app and website, and as far as I'm concerned, it's one of the lowest-priced eSIMs there is. You give them an amount of money to keep in your account, ranging from a low of $10 (I keep $25 in my account), and then they use that. Once it starts to run out, they will pull more money from your credit card that you give them. I used it for 10 days when I was in Norway, since T-Mobile dropped the speed of my data, and the total cost was around $6.40, or something like that.
Another nice thing is that you can use it at home. Recently, our power went out, and when that happens, T-Mobile cell service just goes kaput. I turned on the eSIM and had internet access until the power was restored.