Well firstly i'd avoid those companies who are not transparent, that don't tell you what the virtual office address actually is. They hide part of it and give you a nonsense excuse as to why that is. There is no reason to avoid disclosure of what someone is being expected to pay for up front.
Secondly, make sure they are registered with the ICO for data protection and registered with HMRC for MLR. Both are a legal requirement.
Thirdly, make sure they are not merely re-sellers of the service.
Fourth, consider postage costs. Do the maths and work out what in reality, this would really cost you. Look at what you have to ask that they don't explain/cover on their website, application form or T&C. So the first 20 letters are included for say £15. Great. But what then? Small print might say £1 per letter after that. So what if 5 come in one week? 5 every week?
Some of these companies make their money from their hidden or small print. You only get stung when you go over the quota you saw initially advertised. There is quite often a mark up in the mailing costs for envelopes, handling, packaging, etc. so you pay a higher fee, and not just the cost of the stamp. Tot up all the possible costs across a 12 month period.
Regarding mail coming through, to be fair mail can't really get lost once it arrives at the virtual office address. Royal Mail have more chance of losing it on it's way to the virtual office premises, and again when it's posted on to you. That's where missing mail goes..... into the Royal Mail ether! However, we rarely encounter missing mail. Often it can be down to human error of the client not using/giving out the full correct virtual office address, or the people/organisations they give it to, do not enter the full correct address into their data systems.
You're unlikely to find online reviews because people do not want to expose that they are using a virtual office address, so that's your number 1 reason, and secondly, there won't be enough volume of users to warrant people going online to give reviews.