Most contracts are structured in the way that only the base salary of the first year is guaranteed though (aside of the signing bonus of course). Sometimes the second year of a contract is guaranteed as well, mostly for QBs though.
For example the only guaranteed money Clay Matthews got was the $20.5 million signing bonus.
When considering long term contracts, you have to consider what I would call the additional "tacit guarantees".
Like you said, the only guarantee in Matthews 6 yr. / $70 mil deal was the $20.5 mil signing bonus. That's sounds like a cheap guarantee for this caliber player / contract.
However, for 2014, his dead cap number is $16.4 mil while his 2014 workout bonus + roster bonus + salary bonus = approx. $6.8 mil. The Packers would be strongly incentivized to keep Matthews on IR for the season and pay that $6.8 mil in cash rather than take the approx. $9.6 mil in net additional cap hit in 2014 were he to be released even if he were to sustain a permanent injury. So, given the contract structure, I'd consider that $6.8 mil a tacit guarantee. Teams, agents, players...they're all aware of what they're doing with these contracts and what it implies for future years.
For 2015, Matthews' dead cap number drops to $12.3 mil, while salary and bonuses totaling $8.6 mil. If he were to be released for that season, the net additional cap hit is $3.7 mil. So, there's an incentive to retain him for 2015 as well even if his impact has begun to decline somewhat.
In other threads, rumored Fitzgerald and Ware trades were discussed. Their respective teams are having difficulty affording them (or seeing good value in those players in any case), but their dead cap numbers are so prohibitive as to prevent releasing them. Further, a large dead cap number gives the player considerable leverage in any attempted renegotiation. In a word, these players have their incumbent teams over a barrel. Phoenix or Dallas would have to give me draft picks to take these contracts off their hands in any kind of trade, particularly in the case of Fitzgerald.