Lightsail was created as a cheap alternative to AWS instances. They're the same hardware, just throttled. This allows them to pile more instances onto a single node. Lightsail has a higher amount of instances and throttles them.
If you have an application that bursts quite frequently you can see issues when you hit your burst limit.
PHP is an interpreted language, which means it compiles each time it's run. Every time a PHP request occurs, the code is compiled. Compiling requires CPU, and at times you can see PHP take 100% CPU for a short moment.
Running a WordPress workload on Lightsail is fine, it will run. Especially if your site is mostly cached. But you'll most likely see PHP requests being throttled. An example, WooCommerce stores that are rather active, as logins or checkouts can't be cached. Or a server with a number of WordPress sites, wp-cron and any plugins that run automated processes like backups.
Lightsail is great for applications that don't require large amounts of CPU bursts.