If your destination belongs to a roaming union, a SIM from one member country typically works across the whole bloc at domestic or near-domestic rates, which changes your strategy: buy one good SIM at your first stop and keep it for the entire regional trip. The EU/EEA is the flagship example, where roam-like-at-home rules mean calls, texts, and data cost the same across member states.
The fine print to check in any union: fair use limits, which can cap how much of a large or unlimited domestic allowance you can consume while roaming; the exact member list, since neighboring countries that feel like part of the region are sometimes excluded; and residency-style rules in the EU that expect the SIM's home country to be where you mainly live or travel from, aimed at stopping permanent use of a foreign SIM.
Carriers publish their union roaming terms clearly on their websites.