I am not recommending this route unless you just want to take it. Tamaulipas state has safety issue. PROS
Do I think it is dangerous? Maybe. At the present moment, you are better off taking the Laredo route.
However, if you still want to take this route, just use caution. If you stick to the Gulf Coast, you should be okay. By no means venture inland towards Cd. Victoria or Cd. Mante.
If you are concerned, then by all means, take the Laredo-Yucatan route which is considered "safer" by all. If you'd rather take this scenic route, read on.
Drive to Yucatan and Quintana Roo (Merida, Cancun), Belize & turnoff for Guatemala via the Gulf of Mexico
1. More scenic than Laredo route
2. Few tolls
3. 75 miles shorter than Matamoros route
4. You see small Mexican towns
5. Plenty of RV parks, hotels on / near beaches
6. Few gringo tourists. You'll meed Mexican tourists.
7. Route taken by RV caravans
1. Some consider it unsafe
2. Though shorter, could take longer becasue you transit small towns
3. Lots of topes (speed bumps)
4. Always seems to be 100 KM of bad road somewhere
All 3 routes to Guatemala.