Building a product is hard. Hands down. Keeping up with the latest tech trends, frameworks, databases is more than a full-time job itself. Besides all the code, design, infrastructure to build/manage you are also trying to bring a product to market as fast as possible. To get feedback and iterate quickly. My advice, don't let your tech stack slow you down. Here are some top mistakes I see from new SaaS devs:
1. Trying new or unproven technology
Sometimes a bright shiny framework comes out, makes it to the front page of Hacker News, and suddenly everyone's switching to ShinyNewThing.js. It's all good for a few months until serious projects hit some limitation or design flaw. At that point the project might be abandoned or split into 100s of different forks. One thing is for certain; It will be difficult for you to be productive in this framework.
Experimenting can be fun and learning new technology is definitely important. But when it comes to building SaaS it's generally better to stick to what you know. Your customers, market, or ideas will already need all the development speed and quick iterations you can make in your language/framework of choice.
2. Prematurely scaling
It's easy to get sucked into writing a chef script, k8s config, or whatever else the cool kids are using these days 😎. It's also a great way to not work on your product and open you up to future time sucks. Time is your most valuable asset and prematurely scaling your application, database, or web proxies can really burn up a lot of it.
Now that isn't to say you shouldn't think about tradeoffs you might be making. I recommend leaving yourself a small TODO in your code so you can come back and fix later (if there is a later). You also should make sure your code/stack will scale. Kind of a paradox right?! Well having a great community like a forum, meetup, or conference is a great place to make sure the foundation of your SaaS is stable before you go and build the whole house.
3. Not learning from others
Blazing your own trail can be exhilarating. It can also be terrifying. Perhaps if you are building a complicated novel piece of software it makes sense to not follow others. But in the case of SaaS, almost all of the building blocks so to speak have been done before. Users, logins, email, notifications, design, etc. So much information can be found online for your particular programming language or framework. Most frameworks even have their own subreddit where you can ask your question if you cannot find an answer.
Mistake number 1 makes learning from others more difficult. It's easy to find PHP or Rails devs to ask questions. But not many Cobol programmings building web apps to answer your question. And mistake number 2 is exemplified by having a good community of developers. They can help you pre-maturely scale your app/site if you continue to make that mistake.
Preventing mistakes
Mistakes are gonna happen but with the internet we can do a lot of research and reach out for help when needed. The biggest advantage you have in building your product is the tech community and building on the knowledge you already have.