Yes, unless you cannot conceal it any other way, I would avoid them. They are uncomfortable, you can't take off your jacket or coat in public, and a loaded weapon will likely be pointing at someone anytime you are around people.
In addition, they need to be tied down to your belt to keep them close to the body which increases the discomfort, though may help to hold up your pants.
And finally, unless you are carrying two pistols the load is unbalanced. Even with two loaded mags on the off-side.
I'd have to second Herr Walther. In addition, I think shoulder holsters present retention problems where an attacker / adversary is already near and can close the short distance to grapple. -Keep in mind, most armed confrontations take place within nine feet.
In a shoulder holster, the grip is generally oriented towards them making it easier for them to draw the pistol in a grappling situation than it is for you. Even if the pistol's grip is up, I believe it is far easier for them to draw it from your holster as one pulls back than it is for you to do the same. - YMMV
Andy Arratoonian is a British holster maker, a real craftsman. He has had more than a few customers from the law enforcement community both here and abroad. He has a great guidelines page. Whatever you do or whoever you buy from you should give it a read. (The link is below.) On it, he explains how the use of different styles of holsters is dictated by the function you need and how there is not one right holster for all occasions. When you think about how you sit in a car when driving and you see the holster he has for an armed driver it makes sense.
The p99 has been around for a few years now and there is an ever increasing number of manufacturers an d types of holster for the p99. I currently have three holsters for mine for different situations. I have a Galco shoulder holster for hhen I am dressed in a suit or tux which I can easily draw even if the jacket is buttoned. If it is cooler and I am wearing a sweater or other long top outside my pants I have a Galco paddle holster. For superior concealment under short clothing or a tucked in shirt I splurged and had a custom Matt Delfatti holster made to my specs. I am also left handed but have not found this to be a problem.
Holsters of different types are only useful depending on what you are wearing. Ge used to living with the different types and you will know which to use.Now if it were only so easy to get holsters for my Steyr s40.