I wouldn't say this is a universal "ARM vs Intel" claim, just a specific heuristic for this company's products.

Synology's ARM line is generally underpowered compared to their Intel offerings, in terms of clock speed, memory capacity, and core count. They also tend to gate additional features (e.g. hardware-accelerated encryption) behind their Intel offerings.

edit: We're mostly talking Atoms / Celerons here, so not exactly screaming performance either way.

Unless something has changed in the past year, Docker and Virtual Machines are only supported on their Intel devices and not the ARM ones.

There are third-party package managers for the ARM devices but there are a lot of broken packages so I wouldn't recommend using it.