Unit prices work most of the time but there are a few reasons that I prefer not to use them.
As mentioned in previous posts, different jobs and different companies will yield different costs. This is due to production rates and overhead rates. Coverage rates for materials are basically static. i.e. Paint will usually cover 300 SFPG on a smooth surface unless you are dealing with some coatings with required mils and/or high solids content.
The material cost to put 1 coat of paint on 1 - 3 X 7 door is much greater than the cost for 1 coat of paint on 1 door if you have 7 doors to paint. The reason being is the waste factor. This holds true in most situations until you get to some larger quantities so the size of the job has to play a part in your unit price as well as the total quantity of each substrate.
Here is a larger reason for at least knowing your labor and material requirements.
The source of funding for any contractor is the prices charged on an estimate. It is kind of important that you know everything there is to know about the price that you are charging. If you are developing your own database full of this information then chances are that you know and understand this information. If you are using someone elses data then it is possible that you are not quite as familiar with the pricing that you are using. A database isn't even used unless estimating software is involved and there are many people not using software for their estimating. At least not software that allows you to build a database. For the most part there is no software that works good for someone to take into a person's home to give them a price on some work, especially a painting contractor. This leaves a lot of people out of developing or even using a database.
So many people today hear a unit price that is the current going price for an item in a certain market area. That person will use this number without knowing how much is for material, how much for labor, how much for overhead and for many don't even realize they have overhead or exactly what it is. They have always made money using this number and there is no reason to change. If gas goes up a dollar over the next year maybe they will add a few cents to the unit price. This is guess work and a lot of people do just that.
As far as mistakes and considering that the unit price varies depending on the job size and the substrate quantities then each and every unit price will need to be calculated for that particular job. To do that properly you have to figure labor and materials for the substrate quantities then convert it back into a unit price. That is an extra step and if you aren't doing it this way then you are guessing. That would be more reason to expect mistakes.
I had been estimating a long time and at one point I wasn't getting the amount of work I thought I should. This was years ago and we had charged .35 to paint commercial gyp for years. We always made money and that worked fine until then. One day I went into my boss' office before bidding a job and used .34 instead of .35 on gyp walls. He said "I can't make any money at .34 a SF". My reaction was if you aren't making but .01 per SF for painted gyp walls then there is a huge problem. I would sure expect to make more than $10,000 for painting a million SF of gyp board with 3 coats of paint. At that point in time I couldn't argue those details because I didn't have the facts. Nothing like that has ever happened again since using labor and material to calculate a bid. I still use a unit price as a check for the total price after adding my markups. If you ever have a need to explain something about your price to a customer you won't be guessing at what you are saying.
On a large job with let's say 100,000 SF of gyp to be painted, each penny charged will amount to $1,000.00. If you are wanting to get extremely competitive on a job for whatever reason you could see that even a half a penny would make your total price $500 less. This is often used in commercial construction as a way to help get a job. By reducing a unit price by a half cent, where exactly is the money coming from and what will be your profit by doing this. It may not seem like a big deal but what if you have 500,000 SF to paint. All of a sudden .005 = $2,500.00 less on the total and again it needs to be known where it is coming from. It can't be from material, that price is static, so is your overhead and unless you can talk your painter into going faster the production rate for that job is static too. That money comes directly out of your pocket and it is probably best to know how much will be left if you are willing to let go of that much.
If I want to get extra competitive on a job I will first consider production rates. If the primer and first coat can be applied before the ceiling is installed then there will be no brushwork or "cut in" at the ceiling line then my production about doubles. It is much easier to adjust the production rate for 2 coats of paint than it is to try and figure out how much to reduce a total unit price. If I want to reduce my price more then I have to reduce the amount of markup I am using for profit. Using labor and material keeps all of the guess work out of your prices. It forces you to look at the specifics before pricing. If I were using unit prices most of this would never even occur to me.
This same thing holds true for going up on a price, perhaps a job that you feel you can get a better price for. You can easily add a few cents to your unit price. Now all of a sudden if your painters slow down on the job for whatever reason or there is a problem (bad scheduling etc.) you may never know it because you are still making your standard profit. It wont be until your "extra profits" are eaten up before you know to see what is causing the problem on a job. By calculating labor and material you will know exactly how much time should be required for each coat of paint on each substrate and if a problem arises in the field it will be known long before it starts eating into your profits. The time alotments for a job are also good for giving to field employees on a job so they will know what is expected from them. All of this information is a by-product of calculating labor and material.
If you get a job you will have to calculate material quantities anyway to order them. If you did this on your bid the information is already there.
A good question might be why would a person use a unit price across the board that may or may not work for everything. If it is too high you wont lose any money but you will miss some work. If it is too low then you will get all the work you want but you won't be making the profit you prefer. Using a set price from job to job and company to company is like using a set price for all substrates. It is a guess.
Unit prices work but in order for them to be right and not just a guess, the labor and material of the job needs to be calculated as does your overhead and profit. Using labor and material takes the guessing out of your pricing while giving you better control/knowledge of your field employees production and your expectations of a job.
