We are a Cisco & Meraki partner, and managed services provider. I haven't had anyone express concerns or shortcomings with the Meraki line over Dell's Sonicwall. I would need more specifics to really dig in though.
The biggest items you want to take note of is the licensing. You're going to want to make certain that you purchase through an authorized partner first. Then make certain that you obtain the "Advanced Security" license for your MX. This enables all the robust functionality that you'll want. Tying in with Meraki switches and Wireless Access Points will tighten up security tremendously. Of course your configuration is the next biggest area of concern. Umbrella can be obtained a few different ways. I'd recommend working with your on a complete solution, or getting a good path defined for your planning & upgrades.
Let me know if you have anymore questions... I'm a solutions architect guy but understand the licensing requirements and programs. I'd have to loop in a colleague for any sales details though.
Enjoy!