Any marketing company that guarantees anything other than that they will complete your project is lying to you.
In advertising, all platforms are auction-based which means they are very fluid when it comes to results and can be swayed from a competitor adjusting their bids, budget, ad design, landing pages, SEO, and so much more.
To claim they can guarantee anything other than performing the service is wrong. That being said we do get our clients incredible results which can be seen on our case studies. This is due to the techniques we employ on all client accounts.
If we get the advertising budget we need, and our client is willing to adjust budget as needed so long as its profitable to do so, we can get them better and better results.
People that do not want to put up a proper budget initially will cause their accounts to go through a prolonged learning phase because we will not get the amount of data necessary to quickly optimize the account and start making them good money.
For instance, starting to advertise with minimal budget, on a new account, at the beginning of your busy season, will likely result in not having a well-optimized account well into your season. This can cost you leads and make it more difficult to get all you could have gotten if we were provided a proper budget (think data) to begin with.
For this reason, we typically recommend your account be able to get 10-20 clicks per day at a minimum so we can more quickly optimize your account and get you out of the learning phase.