Any downside to creating web 2.0's (subdomain) on my own domain rather than buying new domains

I'm building a lot of different sites, totally unrelated, and I figured I could save money by using one of my domains to create a "user-submitted blog service" like Wordpress.com, and instead I'm just creating a bunch of different blogs on subdomains totally unrelated to save money on domains and do experiments like AI content, syndicated duplicate content with canonical tags, some original articles, etc.

Is there a downside to using subdomains on my own domain essentially creating my own web 2.0 platform, rather than buying a new $0.99 cent .xyz for each individual project instead of the domain?

Investing a couple of bucks for a domain for a testing project is the least that you can do,
At least, if you really want to make something up & serious with the testing,
Going with a single domain subdomains can do the work, but you'll never end with some accurate results as the domain can fall under spam for some different current or older experience or can simply gain some authority/trust and make another work
Overall, it can be worth the shot for some minus tests

It would be hard for me to differentiate what caused what effect if all my domains where sub domains on one site. $1 for a year domain would be worth it for me to avoid the headache