You start using GCP/AWS/Azure, you're going to start using managed databases, security groups, etc. These are really nice. They can save you a lot of time as a startup. ... They are incredibly difficult to migrate. If you use terraform, it does support GCP/AWS/Azure/DO/Vultr, but you'll have to rewrite everything for each provider.
For really simple providers (just a VM; in AWS, just EC2) you can still write all your own Ansible/Puppet/Chef (I recommend Ansible) to setup your servers for you. You can do your own databases, but there is complexity in scaling, multi-read only workers, etc. Managed solutions are nice in how they handle that for you and you really only need to do off-site backups. But the advantage is, once you have it all written and figured out ... you can move it anywhere.
As a startup, you want to get everything fast. So you're going to get locked in (most likely). That's fine if you start making money. If you want to start cutting costs later, it's not really going to matter who you originally started with. You're going to be rewriting a lot.
There are a TON of tradeoffs going in either direction. Linode/Vultr/DO really appeal to people self-hosting or startups that have infrastructure people from day one who can stand up things, platform-independent, from day one.
DO has started offering managed databases and load balancers. Now we see Linode offering DDOS (maybe saving you money from paying CloudFlare)? Everyone wants to get to the point where they can at least offer the minimum AWS/GCP/Azure stack (web + DNS + load balance + firewall + database .. maybe throw in some managed k8s like DO is doing now?)
It's really all about tradeoffs. What time do you want to put in now so it's easier to migrate later?