It's a difficult one to answer because Hubspot is a continuously evolving platform. What isn't working so well today will be great tomorrow. The speed at which they are developing is mind blowing. I've been watching this now for 1.5 years and it's pretty amazing.
They're doing some pretty cool stuff and constantly pushing the boundaries, I wouldn't want to single any one thing out, none that might not be 100% of the parts will remain so. I just don't know in which order they develop but I know they have a team behind every aspect and like I said keep evolving the product.
So if the question comes of whether it'll be a problem because perhaps there is something that another software might be better in, it won't be long before that's no longer true. Buying into Hubspot means buying into constant change and evolution - that by itself could be a bit of an issue for some people but in my books that's their greatest plus, despite the constant learning curve.

Although my answer doesn't revolve around a particular feature that is vulnerable, I think my answer will still prove helpful.

Hubspot is weakest concerning their pricing. They essentially ignore small businesses with their high pricing structure.

Mike Volpe, their CMO, basically admits this during his interview with me: https://www.growthhacker.tv/mike-volpe

That being said, I think that they are incredibly wise in regards to their pricing. Big companies have money. Big companies require less customer support. And higher margins allow them invest in the future instead of scraping my now.

Still, this is their weakness. It was also the weakness of KISSmetrics and Mixpanel appeared.

I did an appraisal of HubSpot and wrote it up as a slide deck
http://www.slideshare.net/rcaroe/hubspot-demo-pros-and-cons

It shows where alternate offerings can be used (many free). These are where Hubspot is most vulnerable to competition.

Happy to discuss further

To understand which parts of HubSpot are vulnerable to competition you must look at the benefits that the competitors of HubSpot are providing that is what HubSpot is lacking.
1. Marketo: Marketo is often known to be HubSpot's top contender, as they both offer similar features and have high customer satisfaction rates. When you sign up, you are assigned a consultant who will help you with the onboarding process and integration of the system into your company. Marketo offers five engagement marketing solutions: email marketing, lead management, consumer marketing, customer base marketing, and mobile marketing. Their pricing is based on which solution you choose and the size of your company. Marketo is used mostly for larger enterprises with a high number of leads, a designated IT team, and a reasonable budget. It is a great option if you need extensive analytics tracking and workflow streamlining.
2. Infusionsoft: Infusionsoft is a great option for small B2C companies. It is an affordable marketing automation software alternative to more expensive options like Marketo. It has great customer relationship management capabilities, which even includes invoicing and payment tools. Infusionsoft is not known to be great for B2B companies. It is best for smaller B2C companies, especially those with an ecommerce base. Their Premier option even has an online storefront that allows you to sell products, subscriptions, and services.
3. Adobe Marketing Cloud: Creatives who are proficient in Adobe swear by it, and there is a reason: their software really is the best of the best. Their marketing automation, Adobe Marketing Cloud, is obviously expected to follow suit, and for a lot of people, it does not disappoint. It is known to have a slick, user-friendly design, as most Adobe products do. They offer eight solutions: Analytics, Audience Manager, Campaign, Experience Manager, Media Optimizer, Primetime, Social, and Target. Adobe Marketing Cloud is best for large companies or creative individuals who already know their way around Adobe software, like photographers, videographers, and designers. It is not recommended for small companies who do not have a designated marketing team or experience with Adobe.
4. Act-On: Act-On is a strong contender against both HubSpot and Marketo, but it does not have the big price tag attached to it. They aim to provide the kind of all-in-one automated marketing platform that HubSpot does, including website hosting, email marketing, analytics, lead generation, social media management, and more. Users have said that it helped them organize their marketing and campaign tactics to optimize efficiency and get the most out of them, and the interface is known to be very user friendly. Like HubSpot, Act-On also features an abundance of educational resources to help companies and clients learn about marketing. This includes their online learning platform, Act-On University, a resource library, video learning options, and free automation guides and tools. Act-On is great for companies of all sizes, but especially those who are looking for a more budget-friendly option.
5. Wish pond: Wish pond is the new kid on the block, but they are making a big impact. They feature four marketing tools in one powerful platform: marketing automation, landing pages, pop-ups and forms, and contests and promos. The key phrase on their homepage describes them well – “Marketing tools designed to quickly and easily scale a business. No coding required.” The major drawback is that Wish pond doesn’t provide a platform to host your website, so you’ll need to have one already built through a service like WordPress or Squarespace, in which you can integrate all their landing pages, forms, and other tools.

Their website provides a full list of the third-party providers they integrate with, organized by category, so you will know right away whether you will be able to use your other business tools with them. Their pricing plan is a little different, as you can choose the “do it yourself” option, using their marketing suite to build landing pages, create campaigns, and design contests and marketing emails. Or you can choose the “done for you” option, in which Wish pond’s marketing team handles the whole thing – designing and running a marketing campaign to help grow your business. Wish pond is great for companies who are looking for a budget-friendly option and have limited IT knowledge.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath