I've been on 30 day sim only contracts the last few years.

Bad parts: it's sim only so no upgrades and you supply your own phone. Unlocked phones are expensive (but usually cheaper than the equivalent 24 month contract).

Good parts: 30 day contract, cheaper and often better contract terms. Currently with emobile (meteor) and for €25 per month I get unlimited calls & texts and 5GB of 4G data per month. If you buy network unlocked phones you can jump network at any time.

If you can afford the unlocked phone it usually works out a lot cheaper than signing up for a phone+contract.