We have been using Shopkeep for over two years. I chose them because it integrated with App Card (Our rewards system). Like you, I don't love it, but I also don't hate it. It certainly has upsides to it.
I have used a couple of other solutions and the same was true. Just OK for me.
Shopkeep is always up, however, the app card app is so-so.
I also use their payment solution because I was unable to get a better rate elsewhere and it was just easily integrated.
That said, and I don't recall this being an issue elsewhere, the connection to the payment gateway has gone down two times in 2 years. They are generally for a short period of time but the last one was for an hour and it was during our after dinner rush.
Regarding Shopify, I usually disagree with Jesse on this. You will find yourself giving Shopify more money than if you were selling, say, shoelaces. Shopify doesn't like tobacco products so you can NEVER use their payment service. Meaning, you will need to find your own, PLUS pay points to Shopify. The math is easy to do on this. I feel I can speak to this because we are currently setting up a site for E-Juice sales. We go into it knowing we will be paying a bit more but its what our developer uses and he's our partner . But I would never use it for a brick and mortar when there are so many other options out there. These are two different companies so I am not concerned with having two merchant account.
Just a final note on scanning in Shopkeep. We have all the equipment to do it and started using it but quickly stopped when we realized that not all products have barcodes on them. There are more now but you will find that many of the juices out there still do not have them. We created our own barcode schema and purchased the printer and in the end, it was all for nothing. Way too much work to make it worth it. Shopkeep is pretty good with finding products with just a few letters typed into the search. Think hard about naming products and ALWAYS use "Variants" when adding new products. Its easier to find products and makes the whole process simple.
If you have any other questions about shopkeep and making it work for you, let me know. If I was going to start a second business and needed a POS system, I might use it again, or I might not. It's just one of those things. But I would look hard before I did.