Why the word alone tells you nothing
Because no law defines or requires a "guarantee" in this context, an agency is free to use the word to mean almost anything it chooses: a free replacement within a set number of weeks, a partial refund, a re-matching process, or something narrower than any of those. Two agencies advertising "a guarantee" aren’t necessarily offering the same thing, and there’s no regulator checking that the term is used consistently.
This isn’t a sign of anything improper on its own; it simply means the word is doing no work until it’s defined. Ask directly what specifically it covers before treating it as a reason to prefer one agency over another.
What to actually pin down
Ask what circumstance triggers the guarantee, whether it costs anything to invoke, what the exact time window is, and whether it’s a replacement worker, a refund, or something else. Get each answer in writing, since a verbal description of a guarantee isn’t enforceable the way a written contract term is.
Separately, understand that a separate, government-defined process exists for two specific circumstances, a worker failing her medical exam or never arriving after approval, which Immigration handles through its own replacement-permit rules regardless of what any agency guarantee says. That process and a commercial guarantee aren’t the same thing, and confusing them is worth avoiding.