It depends on what you're trying to convey, the context (is this conversational? In a legal brief? A letter?) and who's making the statement:. Opposing counsel? Judge? I s the statement being made in a conclusory way, or is this an attempt to strike a conciliatory tone, to be followed with a big "BUT"? Consider the following, and see which comes closest:
1.I understand the basis of your client's complaint [but the law does not provide a basis for relief].
2. I sympathize with your client [and I will rule in her favor].
3. Your client's case is persuasive [but so is my client's case. We should consider settling.]
4. I can relate to what happened to your client and why he is brining the complaint .[but he has offered no evidence that would support his claims in a court of law]
Maybe it's the lawyer in me, but when I see the word complaint and "your client," I'm instantly cautious. If the person making the statement is the opposing counsel, you want to be careful about not making any concessions, even though you are perhaps attempting to strike a conciliatory tone.