When doing work, I would never want to do job and have it look poorly upon me. So if/when something comes up that will compromise the outcome of the job I tell the owner immediately.
With respect to your painter or the guy you had repairing the soffits. Both jobs you need to be working with good material otherwise if you just do the job you're only "putting lipstick on pig" and not dealing with the real issue, in under a year the job will look like crap and the owner will be unhappy.
On the job it's almost a "no-win" situation, if you say there's going to be more required for the job to be done right they get upset. If you don't do the job correctly and do it cheap, it will also make the home owner upset.
I recommend that you do a walk through of the job you want done with the trades person. Talk about what you want done and ask if in their experience what potential problems could arise.
You want your gutters done, they may have to repair/replace some of the fascia. Ask what's the price for them. Remember that's EXTRA work. There's the getting the wood,cutting it, painting it all before you can do the one job you were hired to do which was install the gutters. Someone that's done the job enough already knows to have the right tools and parts handy in their truck so it minimizes "down time". But there instances where you just run into stuff that you don't expect.
You're tearing down the drywall and you find a nest of carpenter ants and they've eaten into the 2x4 studs. Sloppy work will put up an new sheet of drywall and not treat the issue. Done right you get rid of all the ants, replace the compromised wood and then do the job you were hired to do.
The walk-through you do with the trades person should be like a casual interview. You should be asking a LOT of questions. You should look not only at the fact that they can answer the questions but HOW they answer the questions. Do they look like they're making the answers up on the go or do they really know what they're talking about. Do they seem frustrated with your questions (this is always a big red flag!!!) or are they willing to take the time and go through the questions with you.
If the only question/answer you're concerned about when you get a quote is "how much" then getting caught with unexpected costs are your fault. You are 1 on 1 with a professional and you're not willing to learn. Pros will pick up on this. Many are the type that if you don't ask they're not going to tell. IT's not that they're hiding anything from you, it's just that they're not going to say more than they have to.
I'm always amazed at people who go shopping and will spend 1 or more hours trying to find a pair of pants for $50. These are the same people that will hardly spend 10 minutes doing any research on what needs to be done for maintaining or upgrading a $300K house! Spend an hour or two look up some articles and videos on what job you're getting done, bring in 3+ trades people to give you a quote and ask a boat load of questions! I will assure you you'll have a much more positive experience dealing with contractors and trades people.