This mixer is designed strictly for vinegar and oil... and works beautifully. I've never seen another hand mixer that could create such a stable emulsion. Since it's a plastic agitator, it can be broken, but we use ours at room temperature, or traveling in an ice chest, and have never noted a problem. We do make the first few squeezes of the agitator a bit gentle, until the liquid is moving. I bought my first unit when it first came out. Now we're buying a second unit (for picnics, away from home) from Amazon. About the only kitchen gadget (except silverware and a knife) that we travel with. We've now had our first one for a year.
Yes the cap is small, and we too worry about losing it... but so far no problem and it's otherwise entirely functional.
Yes, it may break if you use it inappropriately (adding things like pieces of stuff, rock salt, extremely viscous liquids, etc. that it isn't made for).
But as an emulsifier for vinegar and olive (or a similarly viscous) oil, not requiring an electric plug, it's the best I've seen since I started looking almost sixty years ago. Five stars... Until someone improves it!