Is an 18mm snap-off knife strong enough for heavy-duty work?

For most cutting, yes. The Olfa 18mm L-1 is cited by long-time tradespeople and crafters in r/Tools as a workhorse for foam, rubber, carpet and cardboard, with a fresh edge available at every snap. For prying or heavy lateral force, reviewers prefer a fixed metal-body knife over any snap-off design.