A backhoe can put a lot of stress on a tractor. Tractors aren't necessarily designed for hoes, much like older tractors weren't designed for loaders.
You should take care using the hoe. There's a lot of hydraulic power at the end of long arms that provide a lot of leverage to multiple forces applied to the tractor. The hoe mount makes a big difference to how a tractor is stressed by the hoe. A sub-frame mount distributes forces more evenly across the frame than a 3ph mount.
I have a 6.5' 3ph hoe on a Ford 1710. I like the convenience of the 3ph, the soil is sandy here, and I don't use the hoe regularly. Never the less, I take care not to work the hoe too hard. I wouldn't try to move a big rock in a trench or break a big root. I also try to keep the digging fairly close to the tractor to reduce the leverage and force applied to the tractor (However, not too close or else you end up with you and the tractor in the hole).
I believe that a 6.5' 3ph hoe combined with educated operating technique is OK for my tractor, and it also gets the work I done that I need to do. However, I do have to remind myself not to get carried away with having TRACTOR POWER. A 1520 is one step smaller than my 1710. If it's a 6' 3ph hoe on the 1520, the tractor is more vulnerable to stress failure than mine and more care should be taken.