Pretty sure you've never been able to learn one type of Engineering, then unlearn and get the other to be able to use both sets of items, they're tied to that specific Engineering(Gnomish Engineering and Goblin). Also https://classic.wowhead.com/item=106...-sapper-charge. "Explodes when triggered dealing 450 to 750 Fire damage to all enemies nearby and 375 to 625 damage to you. (5 Min Cooldown)"

And after a quick google search i ran across this post -https://www.reddit.com/r/classicwow/...ces_ranked_by/. Might be a good idea to take a look at it to get a rough idea of what's available to you with Engineering in Classic.